Gopal went to a fruit market with certain amount of money. With this money he can buy either 50 oranges or 40 mangoes. He retains 10% of the money for taxi fare. If he buys 20 mangoes, then the maximum number of oranges he can buy is

Let's say the amount of money Gopal has is 100x rs. With this money he can buy either 50 oranges or 40 mangoes.

Hence, the cost of 1 orange = $$\dfrac{100x}{50}$$ = Rs. 2x

Similarly, the cost of 1 mango = $$\dfrac{100x}{40}$$ = Rs. 2.5x

Taxi fare = 0.1*100x = 10x

Remaining money with Gopal = 90x
Cost incurred in buying 20 mangoes = 2.5*20 = 50x

Hence, the amount of money left with Gopal = 100x - 10x - 50x = 40x

Therefore, we can say that with this much money he can but a maximum of $$\dfrac{40x}{2x}$$ = 20 oranges.