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.

$\begin{gathered} y\text{ }\propto\text{ x} \\ y\text{ = kx} \\ k\text{ = }\frac{y}{x} \end{gathered}$

Step 2:

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

$\begin{gathered} k\text{ = }\frac{y}{x}\text{ = }\frac{12}{9}\text{ = }\frac{4}{3} \\ \text{Therefore} \\ y\text{ = }\frac{4}{3}x \end{gathered}$

Step 3

You can find x when y = 80.

$\begin{gathered} \text{Frpm y = }\frac{4}{3} \\ \text{When y = 80} \\ 80\text{ = }\frac{4x}{3} \\ \text{Cross multiply} \\ 4x\text{ = 80}\times3 \\ 4x\text{ = 240 Divide both sides of the equation by 4} \\ x\text{ = }\frac{240}{4} \\ x\text{ = 60} \end{gathered}$

Final answer

x = 60