Question

Suppose that y varies directly as x and y=10 when x=8. Find the value of x when y=30

Answer 1

x = 24

30/10 = 3

So, 3x = 24

Answer 2

Varies directly: $\frac{y}{x} = k$

First, solve for k. Then replace the k-value into the equation and cross multiply to solve for the missing variable.

$\frac{10}{8} = k$   ⇒   $\frac{5}{4} = k$

$\frac{30}{x} = \frac{5}{4}$

30(4) = 5(x)

6(4) = x       divided both sides by 5

24 = x

Answer: 24