Help on Maths Exam question?

Let's just say, the 1 litre of water is irrelevant. (9 plants x 1 time a week x 12 weeks) + (11 plants x 2 times a week x 12 weeks) = number of times Joe needs to feed plants. Multiply that by 15 to get how many ml they'll have to use then divide that by 1500 to find the number of bottles needed.