How do you divide 644 by 23?

There are many ways to do it. You could use long division, plug it into a calculator, count multiples of 23 until they reach 644.

One way to work it out is to roughly guess and then correct your answer. For example, here's how I work it out in my head:

I know that 644 is roughly 30 times greater than 23. Not exactly, but roughly. 30 * 23 = 690, and 690 is 46 greater than 644. 46 / 23 = 2, so the answer is 30 - 2, or 28.