let's use engineering notation for the sake of brevity.
1 trillion is 1,000,000,000,000, or just 1E12, twelve zeros.
1 million is then 1E6, six zeros.
we know the discharge for one day is 3E12 gallons, and that'd do just fine for country A for 5 months. How many gallons in 1 month only?

if there are 200million inhabitants in A, namely 200E6 or 2E8 inhabitants, how many gallons per inhabitant from all those 6E11 gallons?

Answer:
P ≈ 0,94
Step-by-step explanation:
The normal deck of cards consists of 52 cards, (with 4 kings )
If we take a king a deck will have 51 cards ( three kings )
The probability to get a non-king card is
Probability of non-king card (P) = 1 - Probability of getting a king card (P₁)
Probability of getting a king card P₁= 3/51 ≈ 0,059
Then P = 1 - 0,059
P ≈ 0,94