If a varies directly as b and a = 3 when b = 12, what is the value of a when b = 18?

Question

what is the answer?

Answer 1

If a varies directly as b then a = kb

The given numbers form a ratio expressed as a/b  = 3/12

To find a when b = 18

We set up another ratio is a/18.

By the definiton both ratios are equal 3/12 = a/18

1/4 = a/18

Cross multiplication

18 = 4a

Divde to each side by 4.

18/4 = 4a/4

4.5 = a

a = 4.5