Let the whole project be 1, and the working efficiency of A, B, C and D, according to the meaning of the question:

a+b= 1/8 ( 1)

b+c= 1/6 (2)

c+d= 1/ 12 (3)

( 1)-(2)+(3)De:a+d = 1/24。

A-D can be completed in 24 days.

Epdm can be used instead of a, b, c and d.

If you don't understand, you can ask.

I wish you happiness.