Test for proportionality - Two dozen balls cost ₹ 1800, 40 balls cost ₹ 3000.

2 dozens = 2 x 12 = 24 units 

Proportion obtained is 24 : 1800 : : 40 : 3000. 

24 : 1800 : : 40 : 3000 can be expressed as $\frac{24}{1800}=\frac{40}{3000}$ 

Transposing the denominators to the numerators 

Hence, $24 \times 3000 = 1800\times 40$

 7200 = 7200 

Since, LHS = RHS 24 : 1800 : : 40 : 3000 is proportional.