So you know the pool has 10000 litres when full.

Water comes in at V=20m liters per minute "m", so after 1 minute (m=1) 20 more litres are added . After 10 minutes [V=20(10) = ] 200 more litres are added.

That's one thing. But the pool is outside and the day is sunny so the Sun is evaporating some of the pool water. This happens at E=0.1m litres per minute "m". So with or without the hose water, water is leaving the pool. So after 10 minutes (m=10) the total water evaporated would be [E=0.1(10) =] 10 litres.

So the first question is easy. How much water will the pool have, at any given moment, in terms of both equations V (for filling) and E (for evaporating).

V=20m E=0.1m

P(m) = 10000 + V(m) - E(m)

That's it. Remember the initial 10000 as the pool already has water.

The second part is also easy, is just the same formula as above, but replace V(m) and E(m) with their respective equations. So:

P(m) = 10000 + 20m - 0.1m P(m) = 10000 + 19.9m