1. The problem is to understand when one number is divisible by another without leaving any remainder. 2. The rule is: If you divide one number by another and the remainder is zero, then the second number is a factor of the first. 3. For example, when you divide 8 by 4, the result is 2 and the remainder is 0, so 4 is a factor of 8. 4. But when you divide 5 by 3, the result is 1 and the remainder is 2, so 3 is not a factor of 5. 5. In simple words, a factor is a number that divides another number exactly, without anything left over. 6. To check if a number $b$ is a factor of another number $a$, divide $a$ by $b$ and see if the remainder is zero: $$a \div b = q \text{ with remainder } r$$ If $r = 0$, then $b$ is a factor of $a$. 7. This helps us find factors and understand divisibility easily.