Because it takes three days for the master to finish it alone and six days for the apprentice to finish it alone,

That is, the master does 1/3 every day and the apprentice does 1/6 every day.

Therefore, the cooperation between two people must be completed within1(1/3+1/6) =1(1/2) = 2 (days).

(2)

Judging from the first question, the master does 1/3 every day, and the apprentice does 1/6 every day. The apprentice does it for three days first, and then two people cooperate, so it is necessary.

(1-1/6 * 3)/(1/3+1/6) = (1/2) =/kloc-0.

So the two need to cooperate 1 day.

I hope it helps you.

I wish you progress in your study! ! ! ! !