As per question:

Expenditure = Rs. 1,200/-

Changes: Price increase of 25%, = 1200 × (125/100) = 1500/-

Person buys 6 kg less.

Suppose person buys x kg of rice at Rs. 1,200/-

Hike lead to reduce the quantity (x -6) kg.

Difference in both price is of 300/-

That means 6kg rice is of 300/-

⇒1 kg = 50/- 

Rs. 50 is price after hike and orginal price is asked.

⇒ 125% = 50

⇒ 100% = 50 × (100/125) = 40

∴ Thus orignial price of rice per kg was Rs. 40/-.