Mary's ticket costs $2/ticket * 1 ticket = $<<2*1=2>>2
Mary's children's tickets cost 3 tickets * $1 = $<<3*1=3>>3
Together the tickets cost $2 + $3 = $<<2+3=5>>5
After paying with a 20-dollar bill, Mary will receive $20 - $5 = $<<20-5=15>>15
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