Triangle ABC has sides 8 cm, 12 cm and 16 cm…

Step 1. Match the sides. Shortest with shortest, middle with middle and longest with longest: $8 \leftrightarrow 10$, $12 \leftrightarrow 15$ and $16 \leftrightarrow 20$.

Step 2. The ratios. $\dfrac{10}{8} = 1.25$, $\dfrac{15}{12} = 1.25$ and $\dfrac{20}{16} = 1.25$.

Step 3. Conclude. The three ratios are equal, so $\triangle ABC \sim \triangle PQR$ by SSS.