Since the crews need to make a total of 750 pies, and the first crew made 235, the second one made 275, then the third one has to make "X" pies.
We can write then the following math equation to solve for the unknown:
235 + 275 + X = 750
combine the numbers on the left of the equal sign
510 + X = 750
now subtract 510 from both sides of the equal sign to isolate "X"on the left
X = 750 - 510
X = 240
So the third crew has to make 240 pies to get to the total that are needed.