(1) The numbers on the calendar are all seven in a row, and the smallest number is X, so the four numbers framed by a 2×2 square should be X; x+ 1； x+7； x+8。

So there is the equation x+8=3x+2.

The solution is x=3.

So these four numbers are 3,4, 10, 1 1.

(2) Party A's work efficiency is 1/20, and Party B's work efficiency is X.

There is an equation1/20+x =112.

The solution is x= 1/30.

After 7 days of cooperation, Party A and Party B still have 5/12,5/12 ÷1/30 =12.5 (days).

So it still takes 12.5 days.

(3) There are N×5/9 people in chemistry and N×4/9 people in physics.