The only gift certificates that a certain store sold yesterday were worth either $100 each or $10 each. If the store sold a total of 20 gift certificates yesterday, how many gift certificates worth $10 each did the store sell yesterday?
(1) The gift certificates sold by the store yesterday were worth a total of between $1,650 and $1,800.
(2) Yesterday the store sold more than 15 gift certificates worth $100 each.