Personally, I think it is the easiest to understand such a problem with equations:

1, assuming that the original purchase price is A, and now the profit is reduced by B, because the selling price remains the same, then:

( 1+20%)a =( 1+5%)( 1+20%-b)a

1+20%? -? b =( 1+20%)↓( 1+5%)

? b=6%

Here, 6% is about equal to

2. Let this batch of parts have 2m pieces, the processing speed of A is A, and the processing speed of B is B. If it is known that both parties get M pieces of processing through known conditions, and the time of known conditions is equal, the following two equations can be obtained:

As shown in the figure, I just found that the score formula can't be seen when it is posted.