y=is partly constant and partly varies e.g x=a and when=5,y=8,when x=6y=4
A.find the value of y when x=15
B.find x when y =20
y = a + bx
so,
a + 5b = 8
a + 6b = 4
Solving these, we get b = -4, a = 28
y = 28 - 4x
Now you can answer the questions.