Question

If y varies directly with x and
y=15 when x is 9, what is x when
y=30?

Answer 1

Answer: 18

Explanation:
Y = Xk
15 = 9k
15/9 =k
30 = 15/9 × X
30/1 ÷ 15/9 =X
30/1 × 9/15 = 270/15

Answer 2

x = 18

Explanation:

given y varies directly with x then the equation relating them is

y = kx ← k is the constant of variation

to find k use the condition y = 15 when x = 9 , then

15 = 9x ( divide both sides by 9 )

`(15)/(9)` = x , that is

x =
`(5)/(3)`

y =
`(5)/(3)` x ← equation of variation

when y = 30 , then

30 =
`(5)/(3)` x ( multiply both sides by 3 to clear the fraction )

90 = 5x ( divide both sides by 5 )

18 = x