Answer:
Mass of water,
![m=1.47* 10^(12)\ kg](https://img.qammunity.org/2020/formulas/physics/college/qrlha54kc7sdp33nasige7nfbfos3oiaei.png)
Step-by-step explanation:
Given that,
Surface area of the reservoir, A = 30 km² = 3 × 10⁷ m²
Average depth, d = 49 m
The volume of the reservoir is V such that,
![V=A* d](https://img.qammunity.org/2020/formulas/physics/college/qf02ed04tw411sud7ve038lrxt5gt7nlcr.png)
![V=3* 10^7\ m^2* 49\ m](https://img.qammunity.org/2020/formulas/physics/college/5xpogd2792mtvitpgy2cbsn7w2vhd76olp.png)
![V=1.47* 10^9\ m^3](https://img.qammunity.org/2020/formulas/physics/college/fnqa1es6sadl92p90r2t6rb9nf6bdjnx4i.png)
We have to find the mass of water is held behind the dam. It can be calculated using the expression for density. We know that density of water, d = 1000 kg/m³
Density,
![d=(m)/(V)](https://img.qammunity.org/2020/formulas/physics/high-school/kf1smi5od7jlq9av9xdr79925ff4srf1w9.png)
![m=d* V](https://img.qammunity.org/2020/formulas/physics/college/86b0kplehoumga7vdhogo7x15bdyks5adu.png)
![m=1000\ kg/m^3* 1.47* 10^9\ m^3](https://img.qammunity.org/2020/formulas/physics/college/uzfh9d3fmutc7gf3jtcuv6der4in721u15.png)
![m=1.47* 10^(12)\ kg](https://img.qammunity.org/2020/formulas/physics/college/qrlha54kc7sdp33nasige7nfbfos3oiaei.png)
Hence, this is the required solution.