put two and two together

If you put two and two together, you correctly guess the truth about something from the information that you have. You can replace `put' with `add'.

He never came out and said, `I am Jewish', but after a period of time, I put two and two together, and I assumed he was.

He knew perfectly well who mother had been. He had only to think back to the time he'd dropped in on Grace and seen that book on pregnancy to put two and two together.

If you say that someone puts two and two together and makes five, you mean that they reach the wrong conclusion about something.

Mr Lane's solicitor said after the case that police put two and two together and made five.