There were 10 + 15 = <<10+15=25>>25 girls in the party.
So, there were a total of 15 + 25 = <<15+25=40>>40 attendees.
40 x 2 = <<40*2=80>>80 cans of soft drinks are needed.
So, Mary bought 80/8 = <<80/8=10>>10 boxes of soft drinks.
Therefore, Mary spent 10 x $5 = $<<10*5=50>>50 on soft drinks.
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