Test for proportionality - A heater uses 3 units of electricity in 40 minutes and 9 units in 2 hours.

A heater uses 3 units of electricity in 40 minutes and 9 units in 2 hours 

2 hours = 2 x 60 = 120 minutes. 

Proportion obtained is 3 : 40 : : 9 : 120 

3 : 40 : : 9 : 120 can be expressed as$\frac{3}{40}=\frac{9}{120}$ .

Transposing the denominators to the numerators.

Hence, 

$3 \times 120 = 40 \times 9$

360 = 360 

Since, LHS = RHS 3 : 40 : : 9 : 12 is proportional.