Suppose an illustrative rate is 1 unit of Currency A for 2 units of Currency B. Ten A corresponds to twenty B under that example. Going back from twenty B requires division by two, not another multiplication.
Write the units into the calculation
Label the rate as B per A. Then 10 A × 2 B per A gives 20 B. Keeping the units visible helps show why the reverse calculation uses the reciprocal relationship. The numbers here are invented and are not current market rates.
Separate arithmetic from a real quote
A provider’s offered rate, fees and direction of exchange may change the actual amount received. Do not use the classroom example as a transaction estimate. For a real comparison, read the provider’s current terms and the final quoted amount for the exact exchange.
When studying a money book, underline which currency each figure uses. If the author switches units without saying so, reconstruct the calculation before accepting the conclusion. A precise conversion begins with identifying what one unit buys, not simply finding two numbers to multiply.