Big Mac Index
The exchange rate between two countries can be compared by dividing the price of a Big Mac in one country (in its currency) by the price of a Big Mac in another country (in its currency).  This value is then compared with the actual exchange rate.  If the rate is lower, then the first currency is under-valued (according to purchasing power parity (PPP) theory) compared with the second currency. Conversely, if the rate is higher, then the first currency is over-valued.