Question

The variable Z is directly proportional to X. When X is 5, Z has the value 55.

Answer 1

Z is directly proportional to x, that means :
z = k.x, with k is the coefficient of proportionality:
Let calculate k:
When x = 5, z = 55. Plug in:
 55 = k.5  → k=55/5 ; k =11
The value of z when  = 12 is : z= kx; z = 11(12) and z = 132

Answer 2

Direct variation is of the form y=kx, in this case:

z=kx, we are given the point (5,55) so we can solve for the constant of variation

55=5k  divide both sides by 5

11=k so our equation is:

z=11x, so when x=12

z=11(12)

z=132