a)
[tex]20 \times 15 = \boxed{300 {cm}^{2} }[/tex]
b)
[tex]300 + (12 \times 15) + (16 \times 15) + 2( \frac{12 \times 16}{2} ) = \\ = 300 + 180 + 240 + 2(6 \times 16) = \\ = 720 + 2 \times 96 = \\ = 720 + 192 = \\ = \boxed{ 912 {cm}^{2} }[/tex]
c)
[tex] \frac{12 \times 16}{2} \times 15 = 6 \times 16 \times 15 = 96 \times 15 = \boxed{1440 {cm}^{3} }[/tex]
