30X+( 100-X)x50=3500

Get X=75.

(2) design: buy a type x lamp.

Column: 100-X is less than or equal to 3X.

D: X is greater than or equal to 25.

Since the profit of Type B is greater than that of Type A, buy more Type B..

Type B can buy 75 at most, then Type A can buy 25.

At this time, the profit is the biggest.

The profit is1875 = 75x (70-50)+25x (45-30).