Unit one is to regard a quantity as a whole, no matter what its value is, it is regarded as ten tenths, which is unit one.

For example, road construction can be regarded as a whole.

If it takes X days for A to build this road, then he will build one tenth of it every day, * * * for X days, and X times one tenth. Isn't that the one?

Then his efficiency is 1 in x, and similarly, the efficiency of b is1/y.

Together, they repair it every day (1/X+ 1/Y), and then multiply it by the number of days they cooperate. N is the whole road completed, which is Unit 1. So,

( 1/X+ 1/Y)N= 1

N= 1/( 1/X+ 1/Y)