A boat goes 45 km downstream in 3 hours and 45 km upstream in 5 hours…

Step 1. The two speeds. Downstream $\dfrac{45}{3} = 15\ \text{km/h}$; upstream $\dfrac{45}{5} = 9\ \text{km/h}$.

Step 2. The pair. With the boat speed $x$ and the stream speed $y$: $x + y = 15$ and $x - y = 9$.

Step 3. Solve. Adding: $2x = 24$, so $x = 12\ \text{km/h}$. Then $y = 3\ \text{km/h}$.

Step 4. The lake. $\dfrac{45}{12} = 3.75\ \text{h}$.