Something was omitted.  We could choose the prices of the two kinds of fruit
different and make the answer come out to be any amount between 20 and 30.

Take the case when mangoes and oranges sell for the same price.
Then 20 mangoes and 30 oranges would be 50 pieces of fruit and
he could buy 25 of each for the same amount of money.

In that case the answer would be 25. He could buy 25 of each for 
the same amount of money. 

Take a second case:

Suppose mangoes sell for 70 cents each and oranges sell for 30 cents each.
Then 20 mangoes would cost $14.00 and 30 oranges would cost $9.00.
That means he would have exactly $23.00.
In that case he could buy 23 mangoes for $16.10 and 23 oranges for $6.90 and
that would also add up to $23.00.

In that case the answer would be 23. He could buy 23 of each for 
the same amount of money.

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For a third case, let's keep the oranges at 30 cents each and up the price of
the mangoes to 4 times as expensive as oranges, and see what happens.

Suppose mangoes sell for $1.20 cents each and oranges sell for 30 cents each.
Then 20 mangoes would cost $24.00 and 30 oranges would cost $9.00.
That means he would have exactly $33.00.  In that case he could buy 22 mangoes
for $26.40 and 22 oranges for $6.60 and that would also add up to $33.00.

In that case the answer would be 22. He could buy 22 of each for 
the same amount of money.

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So something else must be given. Otherwise, as you see, the answer
could be anywhere between 20 and 30.

Edwin