How it works
A bank keeps coins safe. In the made-up Bank Garden, the garden adds a little extra to coins left there. That extra is called interest.
In this garden, interest was one new coin for every ten coins. One hundred coins are ten tens, so year one added 10 coins.
In year two the garden held 110 coins, which is eleven tens, so it added 11 coins. The new coins earn interest too.
This garden is made up, so the numbers are easy. Real banks use different numbers, and a grown-up can tell you more.