The Currency exchange rates are usually based on the market supply and demand, but it can also be determinated by the government, it is called fixed exchange rate. It is kind of monetary policy where some countries control and apply a fixed exchange rate.
This would bring you to $160.00. This is because you initially had $20.00, because of the $13 and $7. Then, your family gave you a total of $140.00, and 140 + 20 = 160.
