Question 1142673
The area of the 4 walls is:
{{{ 2*4*3 + 2*7*3 = 24 + 42 }}}
{{{ 24 + 42 = 66 }}}
The area of the ceiling is:
{{{ 4*7 = 28 }}}
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The combined area is: 
{{{ 66 + 28 = 94 }}} m2
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{{{ 94/20 = 4.7 }}} 
4.7 liters of paint are needed