1. **State the problem:** We are given that the speed $v$ of a tsunami in metres per second is equal to the square root of ten times the depth $d$ of the water in metres. 2. **Write the equation:** The relationship can be expressed as: $$v = \sqrt{10d}$$ where $v$ is the speed and $d$ is the depth. 3. **Find the speed when depth is 40 m:** Substitute $d = 40$ into the equation: $$v = \sqrt{10 \times 40} = \sqrt{400}$$ $$v = 20$$ So, the speed of the tsunami is 20 m/s. 4. **Find the depth when speed is 6 m/s:** Start with the equation: $$v = \sqrt{10d}$$ Square both sides to remove the square root: $$v^2 = 10d$$ Substitute $v = 6$: $$6^2 = 10d$$ $$36 = 10d$$ Divide both sides by 10: $$\cancel{10}d = \frac{36}{\cancel{10}}$$ $$d = 3.6$$ So, the depth of the water is 3.6 metres.