He did 5 races in the lake because 10 / 2 = <<10/2=5>>5
He did 5 races in the ocean because 10 / 2 = <<10/2=5>>5
He raced 15 miles in the lake because 5 x 3 = <<5*3=15>>15
His lake races took 6 hours because 15 / 2.5 = <<15/2.5=6>>6
He raced 15 miles in the ocean because 5 x 3 = <<5*3=15>>15
His ocean races took 5 hours because 15 / 3 = <<15/3=5>>5
He spent 11 hours racing because 5 + 6 = <<5+6=11>>11
#### 11