I am passing through a question in which problem is like `I sold a book for $250$ dollars, which resulted in a loss of 50 dollars. So how much loss in percentage. The formula I understand to fit on it is like
(actual change / original amount ) * 100%
so its like (50/300) * 100% and answer is like 16.67% . But I am not sure I am going in the right way or not , because actual amount is the loss not actual amount and sometime problem come when we don't know the original amount like here I assume the original amount to sum up with loss.
Yes, the actual loss is $50$, since the original amount is $300$ and the selling price is $250$. So actual loss is $300 - 250 = 50$.
However, the loss in percentage is precisely what you computed: $$\frac{50}{300} \times 100\% \approx 16.67\%.$$
By describing this as the percentage lost, we write this percent loss as a positive number. If we had been asked to find percent change, then we would answer that the percent changed is $-16.67\%$, to indicate that the change was a loss.