There were some birds sitting on the fence. 23 more birds came to join them. If there are a total of 43 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. 

That means there were at least 1 bird, but we don't know how many more. 

23 more birds came to join them. 

That means that now we have 23 + the original number of birds. So we have 23 + x = 43. 

That means x = 43 - 23 = 20. 

So the answer is 20.