The calculation of the cross course is made for those foreign currencies that are not direct quotations compared to, say, the Russian ruble. This unknown may be, for example, the Argentine peso. His course is not established daily by the Central Bank of Russia, and is calculated by a mathematical method based on the quotes of other currencies.
In order to know how many rubles you have to pay for one peso, you need to choose the currency using the rate of which will be used for all calculations. The easiest and most obvious choice is the US dollar, the currency buy and sell in all countries so that the value of any currency can be expressed in it. In addition to the us dollar it is convenient to use the Euro.
Exchange rates change daily, so you need to find information on the exchange rate of the Argentine peso against the dollar and the ruble to the dollar on one particular date. For example, the peso to the dollar on 06.09.2011 was 4,4333. That is, to buy 1 American dollar, you need to give 4,43 pesos. The ruble amounted at the same date 29,3436, that is, one dollar is worth 29 rubles 34 kopecks.
Now, by simple reasoning, map that single dollar at the same time, 4,4333 Argentine peso and 29,3436 Russian rubles, that is, these values are equal to each other. Use currency signs, in order to differentiate which of the currencies what is a multiplier.

4,4333 ARS=29,3436 RUB.

Arrange the data in different parts of one equation and divide the multiplier standing at pesos. You will get that 1 Argentine peso is worth 6,6189 rubles, that is 6 rubles 62 kopecks.
In order to understand how much a peso is worth 1 ruble, you need to divide both sides of equation by 29,3436. Thus, for 1 ruble to pay 0,151082 pesos, or in other words 100 Russian rubles worth 15 Argentine pesos and centavos 11.