Measurement

$\tfrac12 \times 8 \times 5 = 20$.

There are 100 cm in a metre, so $2.5 \times 100 = 250$.

$2\pi r = 2 \times \tfrac{22}{7} \times 7 = 44$.

$6 \times 4 \times 3 = 72$.

$\pi r^2 h = 3.14 \times 25 \times 12 = 942$.

$\tfrac12(a+b)h = \tfrac12(7+11)(6) = \tfrac12(18)(6) = 54$.