First, it is necessary to determine whether the copper percentage content of the original two alloys is always unknown. Some can be counted directly, and none can be set.

Percentage of 7 kg = percentage of A 3 kg = weight of B cut = X.

The percentage of copper in remelted gold is equal: [ax+b (3-x)]/[x+(3-x)] = [bx+a (7-x)]/[x+(7-x)]

Convert from the above formula to [(a-b) x+3b]/3 = [7a+(b-a) x]/7 = [7a-(a-b) x]/7.

Solve equation x=[2 1(3a-7b)]/[58(a-b)]

Note that this topic means that the cut piece melts with the rest of the other piece.

Time is tight. I wonder if the answer is right or wrong. The solution to the problem should be right. Let's have a look.