How do I sum 4 binary numbers?

Question

I know how to do the simple binary addition, where there are only 2 binary numbers to be summed, but now I am dealing with 4 binary numbers:

 1 1 0 1
 1 1 0 0
   1 1 1
+  1 0 1
--------

How do I sum these numbers?

Answer 1

Seeing as you know how to add 2 binary numbers, here's what you do. First add the first two numbers to get a number x. Then, add the second two numbers to get a number y. Finally, add together x and y.

Maybe this is clearest using parentheses:

1101 + 1100 + 111 + 101 = (1101 + 1100) + (111 + 101)