Marley bakes a rectangular cake that is 20 inches wide, 12 inches long and 6 inches high. she removes the cake from the pan to frost it. how many square inches of frosting does she need?

We know that
surface area of rectangular cake=area of the base+perimeter of base*height 

only the top area is considered for frosting

area of the base=20*12> 240 in²

perimeter of the base=2*[20+12]> 64 in
height=6 in

surface area of rectangular cake=240+64*6> 624 in²

the answer is
624 in² 

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