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 is X when the fees charged by Company A and Company B are the same, 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.