Question

If y is directly proportional to x and y = 108 when x=9, what is the value of y when x =8 and x=7

Answer 1

Y ∝x
y= kx
substitute x and y values
108= k x 9
because you want to find k on its own you divide both sides by 9
108 ÷ 9 = 12
So k = 9
Now that we have 12 we can use this to find y
y = 12 x 8
You just simply times it because that's what y is
so y= 96 when x=8
To find y when x= 7 you do 7 x 12 which is 84
So y= 84 when x= 7