It costs 2 rides x 2 tickets/ride = <<2*2=4>>4 tickets to ride the Ferris wheel twice.
It costs 3 rides x 5 tickets/ride = <<3*5=15>>15 tickets to ride the roller coaster three times.
It costs 7 rides x 1 ticket/ride = <<7*1=7>>7 tickets to ride the roller coaster seven times.
In total Dolly needs 4 tickets + 15 tickets + 7 tickets = <<4+15+7=26>>26 tickets.
Dolly needs to buy 26 tickets - 20 tickets = <<26-20=6>>6 more tickets.
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