Question 1190234
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1 foot = 12 inches
4 feet = 48 inches (multiplied both sides by 4)


The tank is 48 inches by 21 inches by 13 inches
The volume is 
length*width*height = 48*21*13 = 13,104 cubic inches


Since
1 gallon = 231 cubic inches (exact)
this means we divide by 231 to convert from cubic inches to gallons


13,104 cubic inches = (13104/231) gallons
13,104 cubic inches = <font color=red>56.72727 gallons approximately</font>


If you want to round to the nearest whole gallon, then it would be 57 gallons.


The conversion factor for cubic inches to gallons is something you either memorize, or have on a reference sheet somewhere.
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