(2) Let the expression before the price reduction be y=kx+5, and substitute the point (30,20) into 20 = k * 30+5 to get k = 0.5 and y = 0.5x+5.

(3) Selling 30 kilograms of potatoes before the price reduction, the amount obtained is 20-5 = 15 yuan. So the unit price is 0.5 yuan.

(4) After the price reduction, the price of potatoes sold is =26-20=6 yuan, and the unit price after the price reduction is 0.4 yuan, so the potatoes sold are15kg.

A * * * potato15+30 = 45kg.