Question

If y varies directly as x, and x =9 when y =15, Find y when x =21

Answer 1

y = x

15 = 9

y = 21

9y = 315

y = 315/9

y = 35

Answer 2

35

Explanation:

y is propotional to x, means, if x changes, y changes.

Considering, y depends on x only. Introduce a constant.

y = kx

When x = 9, y = 15,

=> 15 = k(9) => 5/3 = k

When x = 21, note that k is constant.

=> y = kx = (5/3)*21

= 35