An online ticket vendor and a box office sell tickets for a concert. The online vendor sells adult tickets for $18.00 each and child tickets for $12.00 each. A certain purchase of adult and child tickets would cost $216.00 from the online vendor. The box office sells adult tickets for $20.00 each and child tickets for $10.00 each. The same purchase of adult and child tickets would cost $220.00 from the box office. How many child tickets are in this purchase?