[ Arithmetic. Mental arithmetic, etc.]

Ten people wanted to found a club. For this, they need to collect a certain amount of membership fees. If there were five more organizers, each of them would have to contribute 100 dollars less. How much did each person contribute?
Try to represent the condition of the problem as a system of equations. Think about how to solve this problem without forming a system of equations.

## Solution

Let's denote the amount of the entry fee by $x$. Then we can form the equation $10 x=15(x-100)$, solving which, we determine $x=300$ dollars. We could also solve this problem without forming an equation, reasoning as follows: the 100 dollars that the 10 original club members save will be paid by 5 new members, i.e., each of the 5 will pay 200 dollars. Thus, with 15 club members, the total contribution will be $(200 \times 15) = 3000$ dollars. Therefore, for 10 participants, the membership fee was equal to (3000 : 10) = 300 dollars.

## Answer

300 dollars.