Hi
I will guide you through the steps. Please do the calculations by yourself.
Let's first write down what's given:

1) Amount of water used from March to June.
You have correctly written the amount of water used in each month based on the graph.
March : 400 cu.m.
April: 250 cu.m.
May: 300 cu.m.
June: 450 cu.m.

2) Cost of water =
85 cents per cu.m. = $0.85 per cu.m.
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a) Average amount paid per month
= (Total amount paid from March to June) / Total number of months

= (Total amount of water used from March to June x 0.85) / 4

b) Average amount of water used from May to July = 400 cu.m.

= (Total amount of water used from May to July) / Total no. of months

400 = (300 + 450 + y) / 3

where y = Amount of water used in July

Solve for y.

Good luck!