# PSAT Math : How to use the direct variation formula

## Example Questions

### Example Question #1 : How To Use The Direct Variation Formula

Phillip can paint  square feet of wall per minute. What area of the wall can he paint in 2.5 hours?

Explanation:

Every minute Phillip completes another  square feet of painting. To solve for the total area that he completes, we need to find the number of minutes that he works.

There are 60 minutes in an hour, and he paints for 2.5 hours. Multiply to find the total number of minutes.

If he completes square feet per minute, then we can multiply by the total minutes to find the final answer.

### Example Question #6 : Variables

The value of  varies directly with the square of and the cube of . If  when and , then what is the value of  when  and ?

Explanation:

Let's consider the general case when y varies directly with x. If y varies directly with x, then we can express their relationship to one another using the following formula:

y = kx, where k is a constant.

Therefore, if y varies directly as the square of x and the cube of z, we can write the following analagous equation:

y = kx2z3, where k is a constant.

The problem states that y = 24 when x = 1 and z = 2. We can use this information to solve for k by substituting the known values for y, x, and z.

24 = k(1)2(2)3 = k(1)(8) = 8k

24 = 8k

Divide both sides by 8.

3 = k

k = 3

Now that we have k, we can find y if we know x and z. The problem asks us to find y when x = 3 and z = 1. We will use our formula for direct variation again, this time substitute values for k, x, and z.

y = kx2z3

y = 3(3)2(1)3 = 3(9)(1) = 27

y = 27

The answer is 27.

### Example Question #2 : Direct And Inverse Variation

In a growth period, a population of flies triples every week. If the original population had 3 flies, how big is the population after 4 weeks?

Explanation:

We know that the initial population is 3, and that every week the population will triple.

The equation to model this growth will be , where is the initial size, is the rate of growth, and is the time.

In this case, the equation will be .

Alternatively, you can evaluate for each consecutive week.

Week 1:

Week 2:

Week 3:

Week 4:

### Example Question #2 : How To Use The Direct Variation Formula

varies directly as  and inversely as  and . Assuming that no other variables affect , which statement is true of  concerning its relationship to  ?

varies inversely as .

varies directly as .

varies directly as the fifth power of .

varies inversely as the fifth power of .

None of the other statements are correct.

varies inversely as .

Explanation:

varies directly as  and inversely as , meaning that for some constant of variation

.

Setting  and , the formula becomes

Setting  as the new constant of variation, the variation equation becomes

,

so  varies inversely as .

### Example Question #3 : How To Use The Direct Variation Formula

varies directly as both  and the cube of . Which statement is true of  concerning its relationship to ?

None of the other statements is true.

varies inversely as the cube of .

varies inversely as the cube root of .

varies directly as the cube of .

varies directly as the cube root of .

varies inversely as the cube root of .

Explanation:

varies directly as both  and the cube of , meaning that for some constant of variation ,

.

Take the reciprocal of both sides, and the equation becomes

Take the cube root of both sides, and the equation becomes

takes the role of the new constant of variation here, and we now have

so  varies inversely as the cube root of .