Question

Variable y varies directly with variable x, and y = 5 when x = 8.

What is the value of x when y = 15?

Answer 1

8 _ _ _ _ _ 5 
x _ _ _ _ _ 15

8 / x = 5 / 15 ;
x = ( 8 × 15 ) / 5 ;
x = 120 / 5 ;
x = 24 ;

Answer 2

y = kx

Plug in what we know:

5 = k(8)

5 = 8k

Divide 8 to both sides:

k = 0.625

Plug this back into the equation along with y = 15:

y = kx

15 = 0.625x

Divide 0.625 to both sides:

x = 24