Multiplying 11 by double digits

A few weeks ago, I was working with a lad, he’s brilliant, he’s just finished his GCSEs. We had just a couple of minutes left of the lesson, so we decided on a game of times-tables-noughts-and-crosses.I pulled up the grid and asked what he’d like to multiply by. He answered 11.Considering he was doing the higher paper I agreed but thought I’d ‘be clever’ and make the numbers in the grid double digits to make it harder.But he was still answering them within a flash.I asked if he was using a calculator and his mum, whom I hadn’t realised was also there, said he was just using a trick that he had learned.This was eye-opening to me, so I thought I’d share it with you too: Multiplying single digits by 11 is simple.The digit you are multiplying by 11 is written down twice instead of once:11x3 = 3311x8 =88 And this apparently is how you multiply 11 by a double digit easily:Simply add the 2 numbers you are multiplying by 11 together:12x11(1+2 = 3) and slide the answer (3) between the number you were multiplying 11 by: 132 16x111+6 =7Split the 16 and put 7 in the middle16x11 =176 Another example:32x113+2=5Split the 32 and place the 5 in the middle:352