1. **State the problem:** We need to express the sum of 25.8 volts (V) and 720.0 megavolts (MV) entirely in volts (V). 2. **Recall the conversion factor:** 1 megavolt (MV) = $10^6$ volts (V). 3. **Convert 720.0 MV to volts:** $$720.0\ \text{MV} = 720.0 \times 10^6\ \text{V} = 720,000,000\ \text{V}$$ 4. **Add the volt values:** $$25.8\ \text{V} + 720,000,000\ \text{V} = 720,000,025.8\ \text{V}$$ 5. **Final answer:** The sum expressed in volts is $$720,000,025.8\ \text{V}$$ This means 25.8 volts plus 720.0 megavolts equals 720 million and 25.8 volts.