(1) The profit from the production of type A masks is 0.5x million yuan, and the profit from the production of type B masks is 0.3*(5-x) million yuan.

(2)y = 0.5x+0.3 *(5-x) 1.8≤x≤5

(3)① Simplified as y=0.2x+ 1.5, so to make Y (profit) bigger, the bigger the better, so we should produce as many A caps as possible. Therefore, 50,000 A covers are not produced, and the maximum profit is 25,000 yuan.

(2) If 0.6a+0.8b=5 is completed within a+b days, then a=3 b=4 is satisfied, and the shortest time is 7 days.