###Human: Hally and her friends organized a high school reunion. To cover the cost of a late-night party that they planned for at the reunion, each attendee had to pay $100. After the party, Hally and her friends realized that the total expenses were $500 less than the total contributions. If there were 50 people at the party, how much money was used to pay for the party expenses?, ### Assistant: The total planned cost for the party was 100*50 = $5000
The total expenses for the party are 5000-500 = $4500 
 So the answer is :  4500