( 1)y =( 100-80-n)* a *( 1+0. 1 * n)

(2) According to (1):

y=a*(20-n)*( 1+0. 1n)

=a*(20+n-0. 1n^2)

When n=5, (20+n-0.1n 2) has the maximum value.

The maximum value is 22.5a, which should be satisfactory. .....