An oil merchant wants to make a minimum profit of ₹2,100 by selling 50 litres of oil he purchased at ₹236 per litre. For this, he adds a few litres of duplicate oil whose cost price is ₹180 per litre and sells at ₹250 per litre. How many litres of duplicate oil is needed for this purpose?

Let the number of litres of duplicate oil added = a

Cost price of the oil merchant = 50(236) + a(180) = 180a + 11800

Selling price of the oil merchant = (50+a)250 = 250a + 12500

Profit = ₹2,100

$$=$$>  250a + 12500 - (180a + 11800) = 2100

$$=$$>  250a + 12500 - 180a -11800 = 2100

$$=$$>  70a = 2100 - 700

$$=$$>  70a = 1400

$$=$$>  a = 20

$$\therefore\ $$Number of litres of duplicate oil added = 20 litres

Hence, the correct answer is Option C