A swimming pool had 2.5 million liters of water in it. Some water evaporated, and then the pool only had 2 million liters of water in it.

Answer 1

2.5 - 2 = 0.5

so 0.5 million evaporated

0.5 / 2.5 = 5/25 = 1/5 = 20% = 0.2

20% of the water evaporated.

CHECK:

If 20% evaporated then 80% remains.

80% = 0.8, so 0.8 * 2.5 = 2.0 <--- YES, it checks

20% evaporated