A school is going to make a batch of commemorative books for the fresh graduates of two companies. Company A proposed that each book should be charged with a material fee of 5 yuan and a design fee of 1500 yuan. (1) If there are 400 graduates, the expenses of Company A are

400*5+ 1500=3500 (yuan)

The cost of company b is

400*8=3200 (yuan)

If there are 600 graduates, the cost of company A is

600*5+ 1500=4500 (yuan)

The cost of company b is

600*8=4800 (yuan)

(2) If the number of graduates from Company A and Company B is X, there are

5x+ 1500=8x

The solution is x=500.

That is, when there are 500 graduates, the fees charged by Company A and Company B are the same.

(3) When the number of graduates is less than 500, it is more economical to choose Company B; When there are more than 500 graduates, it is more economical to choose a company.