A milk tea shop sold 50 x 2/5 = <<50*2/5=20>>20 cups of winter melon-flavored milk tea.
They sold 50 x 3/10 = <<50*3/10=15>>15 cups of Okinawa-flavored milk tea.
So they sold a total of 20 + 15 = <<20+15=35>>35 cups of winter melon and Okinawa flavors milk tea.
Therefore, they sold 50 - 35 = <<50-35=15>>15 cups of chocolate-flavored milk tea.
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