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?

20 litres

We have,

The oil merchant wants to make a minimum profit = Rs. 2100

50 litres of oil purchased = Rs. 236 per litre

Cost of duplicate oil = Rs. 180 per liter

Selling price of 1 liter oil = Rs. 250

According to the question,

Let quantity of duplicate oil added be x litres.

According to the question

50 × 236 + x × 180 + 2100 = (50 + x) × 250

= 11800 + 180x + 2100 = 50 × 250 + 250x

= 13900 - 12500 = 250x - 180x 

= 70x = 1400

= x = 20