If the two numbers each have N digits, that's N2 (or N x N) multiplications altogether. In the example above, N is 3, and we had to do 3 2 = 9 multiplications.

“With addition, you do it a year earlier in school because it’s much easier, you can do it in linear time, almost as fast as reading the numbers from right to left,” said Martin Fürer, a mathematician ...