A reduction of 20% in the price of salt enables a lady to obtain 10kgs more for Rs.100, find the original price per kg ?

It is based on inverse proportion or product constancy concept. Reduction in price 20% amount of salt will increase 25%.
25% is equivalent to 10 kg
So, total salt = 10 * 4 = 40
Original price of the salt is, 100/40 = 2.5 kg