do the maths
1. Literally, to do a bit of arithmetic calculation. Primarily heard in UK, Australia, New Zealand, Ireland. Well, let's just do the maths to see if you have enough in your savings account to afford a new car.
2. To do figure out or put together information for oneself. Primarily heard in UK, Australia, New Zealand, Ireland. Do the maths—can't you see that he's just using you to get ahead in the company?