In a flexible exchange rate system, explain why a country whose income grows faster than that of

Question:

In a flexible system, explain why a country whose income grows faster than that of its trading partners can expect its to fall, or the value of its currency to depreciate.
Exchange Rate
The value of one currency for the purpose of conversion to another. Exchange Rate means on any day, for purposes of determining the Dollar Equivalent of any currency other than Dollars, the rate at which such currency may be exchanged into Dollars...