Easy way to convert from one fiat to another without using a calculator

Question

I'm trying to convert from one fiat to another without using a calculator. In my case, I was trying to convert 25 TRY to BGN. Let's assume that 1 TRY = 0.13 BGN. That means 25 TRY = 25 * 0.13 BGN = 3.25 BGN.

How can I calculate that more easily in my mind without using a calculator?

It is basically 25 * 13/100, but not fairly easy to calculate it in mind. How would you do it?

Answer 1

First you have to understand the quote you are given. If you are told the conversion is $1.6$ to $1$, is that $1.6$ of $A$ is the same as $1$ of $B$ or the other way around? That tells you whether to multiply or divide by the rate. Then it becomes a matter of practice and knowing arithmetic facts. If you know that $\frac 18=0.125$ you then have $\frac {100}8=12.5$, which is not far from $13$. You can multiply by $8$ and remove two places instead of dividing by $13$. You may find that easier. It helps to know the inverses of all the numbers from $1$ to $10$. Currency rates do not change that much, so you can look up the rate when you have computation available and come up with an approach for when you do not have it available, but doesn't your phone have a calculator? Is the difference between $12.5$ and $13$, which is $4\%$ important in this setting? The more error you can tolerate, the easier it is to find a simple conversion strategy.

Answer 2

I'm trying to convert from one fiat to another without using a calculator. In my case, I was trying to convert 25 TRY to BGN. Let's assume that the daily rate for 1 BGN is 13 TRY. That means 25 * 0.13 = 3.25.

I'm completely lost, if 1 BGN is 13 TRY then 25 TRY should be $25/13\approx 2$ BGN shouldn't it? I don't know where you got that 100 from. At the time of writing 1 BGN was 9.49 TRY.