This Maths article explains both the mental method and written method for multiplying a 2-digit number by a 1-digit number.

We'll explore how this shortcut works for both two-digit numbers multiplied by themselves, and even for multiplying two different numbers ending in 5.

Multiplying three 1-digit numbers The process of multiplying three numbers together is similar to the process of multiplying two numbers, but it has one extra step.

Schönhage and Strassen predicted that an algorithm multiplying n -digit numbers using n * log (n) basic operations should exist, Harvey says. His paper is the first known proof that it does.

Pick a two-digit number — ahem — at random. OK, so you still have to pick a starting number. To get the next number, take the top digit, add six, and then multiply by the bottom digit.

These hard math problems are tricky, but they'll give your brain a workout—and it's really rewarding when you figure one out!

Then take the two numbers you want to multiply—in the case of the above image, 7 and 8—and put together the corresponding fingers (as shown in the picture to the right).