How to find rate

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ACT Math › How to find rate

Questions 1 - 10
1

At the beginning of a race, a person's speed is miles per hour. One hour into the race, a person increases his speed by . A half an hour later, he increases again by another . If he finishes this race in two hours, what is the average speed for the entire race? Round to the nearest hundredth of a mile per hour.

Explanation

Recall that in general

Now, let's gather our three rates:

Rate 1:

Rate 2:

Rate 3:

Now, we know that the time is a total of hours. Based on our data, we can write:

This is miles per hour, which rounds to .

2

A new car can travel an average of 63 miles per gallon of gasoline. Gasoline costs $5.05 per gallon. How much would it cost to travel 6,363 miles in this car?

$505

$101

$405.05

$510

$510.05

Explanation

First, find the total amount of gas necessary for the trip. 6363/63 = 101 gallons (easy to see as 63 * 100 = 6300 + 1 * 63 = 6363). Then multiply the number of gallons by the price per gallon of gasoline, 5.05 * 101 = $510.05 and is your answer (again, easy to see when 5.05 * 100 + 1 * 5.05).

3

Twenty bakers make dozen cookies in eight hours. How many cookies does each baker make in an hour?

Explanation

This problem is a variation on the standard equation . The variable contains all twenty bakers, however, instead of just one. Still, let's start by substituting in our data:

Solving for , we get .

Now, this represents how many dozen cookies the whole group of make per hour. We can find the individual rate by dividing by , which gives us . Notice, however, that the question asks for the number of cookies—not the number of dozens. Therefore, you need to multiply by , which gives you .

4

A motorcycle on a full tank of gas travels 478 miles. If a full tank of gas is 12 gallons, and gas costs $4.25 per gallon, what is the approximate miles per gallon rating of the motorcycle, and how much will a full tank of gas cost?

39.8, $51.00

112.5, $51.00

39.8, $48.75

112.5, $48.75

Explanation

To calculate miles per gallon of gas, we take the 478 miles the motorcycle travels and divide it by the amount of gallons in a full tank of gas, 12 gallons. 478/12 = 39.83, or 39.8 when rounded to the tenths place

For the price, we take the 12 gallons and multiply it by the amount that one gallon costs, $4.25. (12)(4.25) = 51

5

A motorcycle averages 47 miles per gallon. If gas costs $4.13 per gallon, how much gas money is needed for a 1,457 mile road trip?

$128.03

$235.25

$356.06

$570.50

Not enough information given

Explanation

We divide 1,457 miles by 47 miles per gallon to find that 31 gallons of gas are needed for the road trip.

We then multiply the gallons of gas by the cost per gallon to find:

31 x 4.13 = 128.03

6

A climber scrambles over yards of rocks in minutes and then returns across the rocks. If his total rate was yards per minute, how long did it take him to return back?

Explanation

Begin by setting up the standard equation

However, for our data, we know the distance and the rate only. We do not know the time that it took for the person's return. It is , where is the return time. Thus, we can write:

Solving for , we get:

, which rounds to minutes.

7

Sam can paint a house in three days while Dan can finish painting one in two days. How long would it take to paint two houses if they worked together?

0.8 days

2.4 days

1.0 day

1.2 days

None of the answers are correct

Explanation

In general for work problems: W1 + W2 = 1 where Work = Rate x Time

Note, 1 represents the completed job assignment.

For example, W1 is the rate that the first person would finish the job multiplied by the time it would take two or more people to finish the job completely.

1/3x + 1/2x = 1 where x is the time it would take for both people to complete the job.

Find a common denominator to add the fractions, then solve for x.

x = 1.2 days for one house, but the questions asks about two houses, so the correct answer is 2.4 days.

8

Hannah can travel to her destination in one of two ways: she can drive due north for 36 miles, then due west for 44 miles, traveling an average of 65 miles per hour. Or she can drive directly to the destination, heading northwest, traveling an average of 40 miles per hour. What is the difference, to the nearest minute, between the two routes?

11 minutes

12 minutes

16 minutes

20 minutes

24 minutes

Explanation

Remember that distance = rate x time

For the first route, we can set up an equation where the total distance (36 + 44) equals the rate (65 mph) multiplied by the time:

36 +44 = 65t

80 = 65t

t = 80/65 = 1.23 hrs = 1 hr, 14 min

To find the time taken for the second route, we first figure out the distance traveled by using the Pythagorean Theorem.

We know that the "legs" of the right triangle are 44 and 36, where the hypotenuse is the straightline distance (northwest), directly to the destination:

a2+b2=c2

442+362=c2

3232=c2

c=56.85

56.85=40t

56.85/40=t

t=1.42 hrs=1 hr, 25 min

1 hr. 25 min. – 1 hr. 14 min. = 11 min.

9

A farmer has a piece of property that is 10,000 feet by 40,000 feet. His annual property taxes are paid at a rate of $3.50 per acre. If one acre = 43,560 ft2, how much will the farmer pay in taxes this year? Round to the nearest dollar.

$32,140

$3,214

$31,500

$3,500

$35,000

Explanation

Property area = 10,000 ft x 40,000 ft = 400,000,000 ft2

Acreage = 400,000,000 ft2 / 43,560 ft2 per acre = 9,183 acres

Taxes = $3.50 per acre x 9,183 acres = $32,140

10

A large reservoir, holding , has an emptying pipe that allows out . If an additional such pipe is added to the reservoir, how many gallons will be left in the reservoir after three days of drainage occurs, presuming that there is no overall change in water due to addition or evaporation.

Explanation

The rate of draining is once the new pipe is added. Recall that:

, where is the total work output. For our data, this means the total amount of water. Now, we are measuring our rate in hours, so we should translate the three days' time into hours. This is easily done:

Now, based on this, we can set up the equation:

Now, this means that there will be or gallons in the reservoir after three days.

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