A beam 9 m long, 40 cm wide and 20 cm high is made up of iron which weighs 50 kg per cubic metre

(c) 36 kg
Length = 9 m
Breadth = 40 cm = 0.4 m
Height = 20 cm = 0.2 m

$\therefore$ Weight of the beam = volume of the beam $\times$ weight of iron per cubic metre

$=9 \times 0.4 \times 0.2 \times 50$

$=36 \mathrm{~kg}$