Answer:
She asked 10 friends to buy tickets.
Explanation:
If each ticket sells for 3 dollars, you can get the amount of people that bought the tickets by taking the amount earned divided by the price of the ticket.
$18 / $3 = 6
If 6 friends bought tickets and 4 friends did not, add the two together and you will get how much people Monica asked.
6 + 4 = 10
She asked 10 friends to buy tickets.