A river 90 m wide and 2.5 m deep flows at 1.4 m/s…

Step 1. Areas. $90\ \text{m} \times 2.5\ \text{m} = 225\ \text{m}^2$ and $45\ \text{m} \times 3.5\ \text{m} = 157.5\ \text{m}^2$.

Step 2. Speed. $1.4\ \text{m/s} \times \dfrac{225\ \text{m}^2}{157.5\ \text{m}^2} = 2.0\ \text{m/s}$.