GMAT Math : DSQ: Understanding work problems

Study concepts, example questions & explanations for GMAT Math

varsity tutors app store varsity tutors android store

Example Questions

Example Question #21 : Data Sufficiency Questions

Clara spent  of her salary on rent and of the rest on clothes. How much did she have left after paying the rent and buying the clothes?

(1) Her salary was $3000

(2) She spent $1400 on rent and clothes

Possible Answers:

Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.

Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.

D. EACH statement ALONE is sufficient.

Statements (1) and (2) TOGETHER are NOT sufficient.

BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.

Correct answer:

D. EACH statement ALONE is sufficient.

Explanation:

For statement (1), she spent

 

on rent, and

 

on clothes.

So she spent  in total.

Therefore, she has  left.

For statement (2), we can set Clara’s salary to be , then we have

 

Then simplify the equation:

 

Therefore, .

Now we can just follow what we did for the first statement to calculate the money he had left.

Example Question #22 : Data Sufficiency Questions

Steve can paint his greenhouse in 4 hours 40 minutes minutes, working alone; his brother Phil can do the same job in 6 hours, working alone. If they work together, to the nearest minute, how long will it take them? 

Possible Answers:

2 hours 22 minutes

2 hours 38 minutes

2 hours 42 miutes

3 hours 

2 hours 54 minutes

Correct answer:

2 hours 38 minutes

Explanation:

Think of this in terms of "greenhouses per minute", not "minutes per greenhouse". Converting hours and minutes to just minutes, Steve can paint  greenhouses per minute; Phil can paint  greenhouses per minute. 

If we let  be the time in minutes that it takes to paint the greenhouse, then Steve and Phil paint   and  greenhouses, respectively; since one greenhouse total is painted, then we can add the labor and set up this equation:

Simplify and solve:

or 2 hours, 38 minutes.

Example Question #23 : Data Sufficiency Questions

A large water tower can be emptied by opening one or both of two drains of different sizes. On one occasion, both drains were opened at the same time. How long did it take to empty the water tower?

Statement 1: Alone, the larger drain can empty the tower in three hours.

Statement 2: The smaller drain can empty water at 75% of the rate at which the larger drain does.

Possible Answers:

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

EITHER statement ALONE is sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question. 

Correct answer:

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Explanation:

A work problem is actually a rate problem in disguise.

If you know that an object working alone can do a job in  hours, then you know that the object works at a rate of  jobs per hour. After  hours, the object accomplishes  of a job. Similarly, the other object working alone does a job in  hours, and therefore does  of a job. Together, the objects do one whole job, so solve this equation 

for .

Statement 1 alone gives us half the picture; , but  is unknown.

Statement 2 alone tells us that  is 75% of . But  is unknown.

From Statement 1 and 2 together, we know

 is 75% of  - this allows us to calculate :

75% of  is , so . Since we have both  and , we have the complete equation

and we can calculate .

Example Question #24 : Data Sufficiency Questions

A large water tower can be emptied by opening one or both of two drains of different sizes. On one occasion, both drains were opened at the same time. How long did it take to empty the water tower?

Statement 1: Alone, the smaller drain can empty the tower in three hours.

Statement 2: Alone, the larger drain can empty the tower in two hours.

Possible Answers:

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

EITHER statement ALONE is sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question. 

Correct answer:

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Explanation:

A work problem is actually a rate problem in disguise.

If you know that an object working alone can do a job in  hours, then you know that the object works at a rate of  jobs per hour. After  hours, the object accomplishes  of a job. Similarly, the other object working alone does a job in  hours, and therefore does  of a job. Together, the objects do one whole job, so solve this equation 

for .

Statement 1 alone tells us that , and Statement 2 alone tells us that .

Therefore, each statement alone gives us only half the picture, but together, they give us the equation

,

which can be solved to yield the answer. 

Example Question #25 : Data Sufficiency Questions

Three brothers - David, Eddie, and Floyd - mow a lawn together, starting at the same time. How long will it take them to finish?

Statement 1: Working alone, David can mow the lawn in four hours.

Statement 2: Working together, but without David, Eddie and Floyd can mow the lawn in three hours.

Possible Answers:

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question. 

EITHER statement ALONE is sufficient to answer the question.

Correct answer:

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Explanation:

A work problem is actually a rate problem in disguise.

If you know that David working alone can do a job in  hours, then you know that the object works at a rate of  jobs per hour. After  hours, the object accomplishes  of a job. Similarly, Eddie and Floyd, working without David, do a job in  hours, and therefore does  of a job. Together, the objects do one whole job, so solve this equation 

for .

Statement 1 alone tells us that , and Statement 2 alone tells us that ; each one alone leaves the other value unknown. However, if both statements are given, the equation 

can be solved for  to yield the correct answer.

Example Question #26 : Data Sufficiency Questions

Last week, Mrs. Smith, Mrs. Edwards, and Mrs. Hume were able to write  invitations to a party in two hours. 

Today, Mrs. Smith is sick and cannot help, so Mrs. Edwards, and Mrs. Hume have to work without her. They must write  more invitations to the same party. How long should they take, working together?

Statement 1: Working alone, Mrs. Smith can write  invitations in one and one-half hours.

Statement 2: Working alone, Mrs. Hume can write  invitations in one hour.

Possible Answers:

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are insufficient to answer the question. 

EITHER statement ALONE is sufficient to answer the question.

Correct answer:

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

Explanation:

Since the group is now without the help of Mrs. Smith, we look at Mrs. Smith's contribution to the work as a whole, and the sum of the other two ladies' contribution as a whole; Statement 2, which deals with Mrs. Hume alone, is irrelevant and unhelpful.

Since the three ladies together wrote 400 invitations in 120 minutes, we can infer that they would have spent three-fourths of this time, or 90 minutes, writing 300 invitations.

If Statement 1 is true, then Mrs. Smith, who can write 100 invitations in 90 minutes, would take three times this, or 270 minutes, to write 300 invitations. 

A work problem is a rate problem in disguise. Think in terms of "jobs per hour", and take the reciprocal of each "hours per job". The three ladies together would have done  job in one hour, and Mrs. Smith alone would have done  job in one hour.

Therefore, in one hour today, the two ladies will do

 

jobs, and in  hours today, they will do

job.

Solve for  in this equation. 

This proves that Statement 1 alone allows us to find the answer - but not Statement 2.

Learning Tools by Varsity Tutors

Incompatible Browser

Please upgrade or download one of the following browsers to use Instant Tutoring: