Hello!
The answer is:
Andre exchanged 450 dollars for pesos.
Why?
To solve the problem, we need to write two principal equations in order to establish a relation between the number of dollars before and after the trip.
So,
Before the trip, we have:

We have that he exchanged 450 dollars for pesos.
Also, we can calculate how many pesos he had before he exchanged it back to dollars, so:
After the trip, we have:

We have that he had 900 pesos before he exchanged it back to dollars.
Hence, we know that:
Andre exchanged 450 dollars for pesos.
Have a nice day!