3 Up to now we have talked about the sum of two numbers. However, no matter howmany numbers we are adding, we never add more than two numbers at a time.
For example, to form the sum 2 + 3 + 4, we can first add 2 and 3 to obtain 5 and thenadd 5 and 4 to obtain 9. We would obtain the same result if we had first added3 and 4 to obtain 7 and then add 2 to obtain 9.