Converting pesos to dollars and vs versa

Hi,

Lets say (for example) that I convert $1,000 dollars to Mexican pesos
at 11% and end up with $11,000 pesos.

Now lets say I want to convert $11,000 pesos to dollars at the same
rate. The way I am doing it now would be to multiply the $11,000 pesos
x .090 which gives me $990 dollars, I am missing $10 dollars.

How do I figure out what the peso to dollar exchange rate is using the
known dollar to peso exchange rate? I want to change pesos to dollars
at the same rate I changed dollars to pesos.

Any Assistance appreciated. 

Adios for now, 

Daniel Hallas

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