Neha is told that the HCF of 234 and 390 is 78. Find their LCM without factorising.

Step 1. The rule. LCM $= \dfrac{234 \times 390}{78}$.

Step 2. Divide first. $234 \div 78 = 3$, so LCM $= 3 \times 390 = 1170$.

Step 3. A check. $1170 \div 234 = 5$ and $1170 \div 390 = 3$, so both divide it.