7 2/15-(1 5/6+2 1/2)
1) 1 5/6+2 1/2 = 1 5/6 + 2 3/6 = 3 8/6 = 4 2/6 = 4 1/3
2) 7 2/15 - 4 1/3 = 7 2/15 - 4 5/15 = 6 17/15 - 4 5/15 = 2 12/15 = 2 4/5
