(2) Let y=4800, then

(400-x)(8+2x/50)=4800

Solution, x= 100 or x=200.

If we want to benefit the people, we should lower the price of each set by 200 yuan.

(3)y =(400-x)(8+2x/50)=-2 x2/25+24x+3200

y′=-4x/25+24

So, let y'=0.

x= 150

y=5 100

Therefore, when the price of each unit is reduced by 150 yuan, the maximum profit is 5 100 yuan.