The average value of a function y = f(x) on the interval [a,b] is defined by 1/(b-a)∫(from a to b)f(x)dx.
So, if f(x) = 5x4, then the average value of the function on the interval [1,5] is:
1/(5-1)∫(from 1 to 5)5x4dx = (1/4)[55 - 15]
= 781