Question

The value of y is directly proportional to the value of x.If y=12 when x=9, what is the value of x when y=80

Answer 1

Step 1:

Write the formula connecting y and x.

`\begin{gathered} y\text{ }\propto\text{ x} \\ y\text{ = kx} \\ k\text{ = }(y)/(x) \end{gathered}`

Step 2:

Find the value of k when y = 12 and x = 9

`\begin{gathered} k\text{ = }(y)/(x)\text{ = }(12)/(9)\text{ = }(4)/(3) \\ \text{Therefore} \\ y\text{ = }(4)/(3)x \end{gathered}`

Step 3

You can find x when y = 80.

`\begin{gathered} \text{Frpm y = }(4)/(3) \\ \text{When y = 80} \\ 80\text{ = }(4x)/(3) \\ \text{Cross multiply} \\ 4x\text{ = 80}*3 \\ 4x\text{ = 240 Divide both sides of the equation by 4} \\ x\text{ = }(240)/(4) \\ x\text{ = 60} \end{gathered}`

Final answer

x = 60