How to obtain 4 input or gate using 2 input gates?

You have four inputs a,b,c,d.
Run a and b in one or gate. And get output m.
Run c and d in another or gate. And get output n.
Run m and n in a third or gate, and you have your final output z.

 

Doing the problem for him doesn't help him learn. :tongue2: For example, it takes away from him the possibility of figuring out on his own that he could look at the algebraic expression W or X or Y or Z, and figure out that he can group them in pairs, then turn the result into a circuit. These things are much better learned by working it out yourself than being handed the answer.