[ Arithmetic. Mental calculation, etc.]

For the book, 100 rubles were paid, and it remains to pay as much as it would remain to pay if it had been paid as much as it remains to pay. How much does the book cost?
This task is very similar to task 2, only even more complicated.

## Solution

If you read the condition carefully, you will understand that the 100 rubles paid are the "first half" of the book's cost. This means the book costs 200 rubles.

## Answer

The book costs 200 rubles.