They played 15 rounds and William won 5 more rounds than Harry so that leaves 15 - 5 = <<15-5=10>>10 round
There are 2 players and 10 rounds so they each won 10/2 = 5 rounds
William won 5 more rounds than Harry and they each won 5 rounds so William won 5+5 = <<5+5=10>>10 rounds
#### 10