Let say Gopal has $100$ rupees.
$100\:\text{rupees} \implies 50$ oranges
$1\:\text{rupees} \implies \dfrac{50}{100} = \dfrac{1}{2}$ oranges
$\text{(or)}$
$100\:\text{rupees} \implies 40$ mangoes
Gopal gives $10\%$ of the money to the taxi fair, then the remaining amount $ = 100 – 10 = 90$ rupees.
If he can buy $20$ mangoes, then he will spend $50$ rupees.
Now balance amount $ = 90 – 50 = 40$ rupees.
Now, Gopal can buy $40 \times \dfrac{1}{2} = 20$ oranges.
So, the correct answer is B.
