Merry had a total of 50 + 25 = <<50+25=75>>75 boxes of apples.
These 75 boxes is equal to 75 x 10 = <<75*10=750>>750 apples.
There were 750 - 720 = <<750-720=30>>30 apples left.
Thus, 30/10 = <<30/10=3>>3 boxes of apples are left.
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