There were some birds sitting on the fence. 11 more birds came to join them. If there are a total of 20 birds on the fence now How many birds had been sitting on the fence at the start?
There were some birds sitting on the fence. This is telling us that there is an unknown number of birds, which we will call "x".

11 more birds came to join them. This is telling us that there are 11 more birds, which we will call "y".

Now, we have to find out how many birds are on the fence in total. To do this, we have to add "x" and "y" together.

If there are a total of 20 birds on the fence now, this is telling us that "x + y = 20".

Now we have to solve for "x". We can do this by subtracting "y" from both sides of the equation.

"x + y = 20"

"x + y - y = 20 - y"

"x = 20 - y"

Now we have to plug in the value of "y", which is 11.

"x = 20 - 11"

"x = 9"

This means that there were 9 birds sitting on the fence at the start.