Fluctuations in a country's foreign exchange rate is based on the demand and supply of that currency. A country's currency is said to be weak when compared to the global common denomination of exchange (US dollar) when its value has depreciated over a period of time, domestic inflation increases, and imports are greater than the exports.
In this case the currency of country in which the foreign company is located is weak, i.e., to obtain $1, you need to spend 2.57 of the foreign currency. So initially, when the company invested $20 million US dollars in the foreign company, it is equivalent to 51.4 million ($20 mil x 2.57) in the local currency. However, with fluctuations in the foreign currency and a further decline in the value to $3.15, the value of the investment has decreased to US $16.31 million (51.4/3.15), resulting in a loss of $3.69 million. In other words, it can be said that there has been a 18.4% (3.68/20) decrease in the value of the company's investment.