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To find the volume of the tank (of the prism), multiply the flow rate of 1070 liters per minute by 45 minutes.
You will get the volume in liters.
Each liter is 0.001 of the cubic meter.
So, when you find the volume in liters, divide it by 1000 to convert it to cubic meters.
Then divide the found volume value by the area of the base of the prism, which is 7.8*2.9 suare meters.
Happy calculations (!)
If you still have questions, do not hesitate to post it to me.