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To find the volume first figure the size of the rectangle on the end formed if you drew a line across the tank at the top of the flat sides. That value times 60 will give the volume of that much of the tank. You then need to determine the area of the remaining end of the tank. If the remaining area is a half circle then 27 inches is the diameter and the calculations are easy. The size would be half of the circle.
In your case the upper part is most likely less that half a circle. You can measure the distance from a line across the top of the rectangle to the top of the tank. You can then use this dimension to find the area of a segment of a circle.
This site will help you find the formulas you need for the calculations.Area of a circle
You will get pretty close to the volume if you just take this measurement times 13.5 times 60 and add it to the volume of the lower cube.