The purchase rule of a commodity is: no more than 100 pieces, according to B yuan per piece; if more than 100 pieces, according to (b-20) yuan per piece. It costs a yuan to purchase no more than 100 pieces now. If 13 pieces are added on this basis, the cost is still a yuan. If the purchase price is set to be whole yuan per piece, then B yuan=______ ．

The purchase rule of a commodity is: no more than 100 pieces, according to B yuan per piece; if more than 100 pieces, according to (b-20) yuan per piece. It costs a yuan to purchase no more than 100 pieces now. If 13 pieces are added on this basis, the cost is still a yuan. If the purchase price is set to be whole yuan per piece, then B yuan=______ ．

Let the original purchase x pieces, (x < 100), then XB = A and (x + 13) (b-20) = a, two equations are simultaneous, and XB = (x + 13) (b-20) is obtained. Simplify, and get b = 20 + 20x13. From known, get x < 100, and X + 13 > 100, the solution is 87 < x < 100. And B is an integer, and get x = 91, that is, B = 20 + 140 = 160. So the answer is: 160