NMOS 2010 Q7

1/2 x 2 x 6 = 6

1/2 x 3 x 1 = 1.5

1/2 x 2 x 4 = 4

1/2 x 7 x 1 = 3.5

6 + 1.5 + 4 + 3.5 = 15

5 x 2 = 10

2 x 1 = 2

4 x 3 = 12

4 x 4 = 16

10 + 2 + 12 + 16 = 40

40 - 15 = 25 cm 2.