Let's assume that the price of Big Mac in Turkey sells for $20$ Turkish Lira $($TL$)$ and that the price of the same burger is $6$$ in the U$.$S$.$A$.$ Please calculate the TL$/$$ exchange rate according to the law of one price and interpret if the actual exchange rate is overvalued or undervalued, if the exchange rate is at $4$ Turkish Lira per US$$.$ If it is overvalued or undervalued, what causes such a disparity between the actual exchange rate and the assumed exchange rate by the law of one price?