Let X be the number of children in the group.
The number of adults in the group is X+<<+25=25>>25.
The group pays 8*X + 15*(X+25) = $720.
Multiplying through the parentheses, we get 8X + 15X + 375 = $720
Combining like terms, we get 23X + 375 = 720
Subtracting 375 from both sides, we get 23X = 720 - 375 = $345
Dividing both sides by 23, we get X = $345 / 23 = <<345/23=15>>15 children.
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