A bus drives along a straight highway for one hour and twenty minutes…

Step 1. Equal times. In time $T$ at each speed, the bus covers $36T + 54T = 90T$ in $2T$. Average: $\dfrac{90T}{2T} = 45\ \text{km/h}$, the simple mean.

Step 2. Equal distances. Each half of length $d$ takes $\dfrac{d}{36\ \text{km/h}}$ and $\dfrac{d}{54\ \text{km/h}}$ hours, so the average is $\dfrac{2d}{d/36 + d/54} = \dfrac{2 \times 36 \times 54}{36 + 54} = 43.2\ \text{km/h}$.