I want to reflect the value of one number in terms of another.
For example Euros in terms of Dollars.
So in one cell I put the number of Euros and multiply them by the
exchange rate of 1.33 to get the amount in Dollars.
But now I want to put the number of Dollars and use the same exchange
rate to find out how many Euros those Dollars worth.
How can I make a cell to use a formula and also to be able to take a
number without erasing the formula that is in it?

A1 B1 C1

100 Euros 1.33 =A1*B1 which is straight forward

But now I want also to be able to put 100 Dollars in cell C1 and have
the equivalent amount in Euros in cell A1.

Thanks,