Method-algebraic method. 5% ÷1%= 5,4× 5 = 20 (pieces); Assuming that the cost of this commodity is X yuan, from the profit of 100 yuan, we can get:100× 80x =100× (1-5% )× (80+20)-(80+20) x-650.

Method 2: Correspondence method. Reduce the price by 5%, and order 4×5=20 pieces, that is, order 100 pieces. If 5 yuan is subtracted from each piece, a * * * minus 5× 100+ 100=600 yuan. This is the profit of 20 pieces bought in 600 yuan at the original price, that is, the original profit is 600÷20=30 yuan, and the cost 100-30=70 yuan. (LY)