There were some birds sitting on the fence. 16 more birds came to join them. If there are a total of 28 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. 

16 more birds came to join them. 

That means that the total number of birds on the fence is 16 + the number of birds that were already there. In other words, it's 16 + the unknown number. 

If there are a total of 28 birds on the fence now, that means that 16 + the unknown number is 28. 

Can you solve for the unknown number?