The fee of a store is: 5 * 40+(x-5) *10 =10x+150 yuan.

The cost of store B is: (5 * 40+x *10) * 0.9 = 9x+180 yuan.

Store A-Store B fee: y =10x+150-(9x+180) = x-30.

When x> is at 30 o'clock, y>0 is very cost-effective.

When x=30 and y=0, the expenses of store A and store B are the same.

When x