Explanation:

 Where we want to keep the expenditure same, There the Price ratio and Quantity ratio is in reverse order.

Price Ratio:-

100 : 120 => 5 : 6

Quantity Ratio:-

6 : 5

Decreased Value of Quantity => 1 unit ---------> 2 eggs

                                              => 5 units --------> 5* 2 = 10 eggs.

Now 10 eggs are bought in Rs.24.

So New(Reduced) price per egg is => 24/10 = 2.4 Rs./egg

Now the Price of 1 egg = Rs.2.4

Price of 12 eggs = 12 * 2.4 = 28.80 Rs.