$3,500 budget minus $50 clean up fee equals $3,450.
3500 - 50 = 3450
$3,450 divided by $31 per person equals 111.29
3450 / 31 = 111.29
111 people multiplied by $31 equals $3,441 plus $50 clean up fee equals $3,491
111 x 31 = 3441
3441 + 50 = 3491
They can invite 111 people and will have $9 to spare