Question

Given that y and x are directly proportional and y=3 when x=6, find the value of y when x=18.

Answer 1

y = 9

Explanation:

If x and y are directly proportional, then the quotient y : x is constant.

We have:

x = 6 and y = 3

x = 18 and y = ?

We have the equation:

`3:6=y:18\\\\(3)/(6)=(y)/(18)\qquad\text{cross multiply}\\\\6y=(3)(18)\\\\6y=54\qquad\text{divide both sides by 6}\\\\(6\!\!\!\!\diagup y)/(6\!\!\!\!\diagup)=(54)/(6)\\\\y=9`