First video:

Charlie said: "Alison, think of a two-digit number. Reverse the digits and add your answer to your original number. I bet your answer is a multiple of 11."

Alison chose 42, added 24 and got the answer 66: "It is! How on earth did you know that?"

Charlie said: "I'm not sure. Let's try to work it out."

Alison chose 42, added 24 and got the answer 66: "It is! How on earth did you know that?"

Charlie said: "I'm not sure. Let's try to work it out."

Second video:

Alison arranged multilink to show **four tens** and **two units** for 42, and **two tens** and **four units** for 24.

She then put the**four units** with the **four tens**, and the **two units** with the **two tens**, giving six lots of eleven.

Charlie imagined a two-digit number $ab$, where $a$ represents the number in the tens column, and $b$ respresents the number in the units. This can be written as $10a+b$. Similarly, $ba$ can be written as $10b+a$.

Charlie added these together to get $11a+11b$, which he wrote as $11(a+b)$.

She then put the

Charlie imagined a two-digit number $ab$, where $a$ represents the number in the tens column, and $b$ respresents the number in the units. This can be written as $10a+b$. Similarly, $ba$ can be written as $10b+a$.

Charlie added these together to get $11a+11b$, which he wrote as $11(a+b)$.