Generally speaking, everything you ask in an application problem is set as an unknown number, but sometimes if you set the unknown number directly, it will increase the difficulty of the equation. At this time, it is necessary to find a suitable unknown and set it as unknown X or Y, and then get the required unknown through simple arithmetic operation.

For example, two typists, A and B, can finish a manuscript in 24 hours. Now A typed 16 hours first, and then B typed 12 hours, thus completing 3/5 of this manuscript. As we all know, A types 300 more words per hour than B. How many words are there in this manuscript?

If we directly assume that there are x words in this manuscript, it is difficult to list this equation, so we will look for indirect unknowns. If we can know how many hours it will take Party A and Party B to finish this manuscript alone, it will be easy to calculate the number of words in this manuscript. Therefore, in this problem, it can be indirectly set as "It takes X hours for A to complete alone, and Y hours for B to complete alone", and then the equation is listed according to the engineering problem, and the number of words in this manuscript can be found soon.