maths problem

Question

12 ) y is directly proportional to x3 When x = 2, y = 20. (a) Find y when x = 8.
¦ (b) Find x when y = 1000.

Answer 1

y is directly proportional to x means y=kx, k being some constant.

20=k(2)  //comes from the (2,20) pair

k = 10

y = 10x

 

y = 10*8 = 80 when x=8

 

When y=1000=10x, x=100

 

I hope this helps.