1,2,3,4. Daughter confused, see photo

Dropping less than two inches per mile after emerging from the mountains, a river drains into the ocean. one day's discharge at its mouth, 5.6 trillion gallons, could supply all of country a's households for seven months. based on this statement, determine how much water an average household uses each month. assume that there are 100 million households in country a. country a uses approximately nothing gallons per household per month.

Ineed me i stuck and i don’t want to get it wrong

I'm confused on this could anyone me and show me what i need to do to solve it.