How would you work this problom If y equals 25 when X equals 5 what is the value of y when x equals 20?

There is no one solution to this problem. For example:

y = x2

y = 5x

y = x + 20

y = 2x / 5 + 23

These are just four of the infinite set functions that would give "y" a value of 25 when x = 5.