Question

If y varies directly as x and y = 3 when x = - 9, then find x when y = 5.

a. 15
b. 1/15
c. - 1 / 15
d. - 15

Answer 1

d. - 15

Explanation:

The equation for direct variation is

y = kx

We know y =3 when x = -9

3 = k(-9)

Divide by -9

3/-9 = -9k/-9

-1/3 = k

The equation is then

y = -1/3 x

We know y = 5

5 = -1/3 x

Multiply each side by -3

-3*5 = -3*-1/3x

-15 = x