Fractions can be added in different ways: When fractions have the same denominator close denominatorNumber written on the bottom of a fraction. The denominator is the number of equal parts. Eg ...

There's a helpful shortcut worth remembering: if your fractions already share the same denominator, you can skip straight to adding or subtracting the numerators. No LCD hunting required.

Why do you need to find the lowest common denominator when adding or subtracting fractions but not when multiplying them? And why do you have to invert the second fraction and multiply when dividing?