20 2/7 = 142/7 -20/35
find common denominator ( in this case it is 35)
35/7 =5 , multiply 142*5=710, 7*5 =35
142/7 = 710/35
710/35 - 20/35 = 710-20 = 690
690/35 now simplify that fraction
690/35 = 19.714
19*35 = 665
690-665=25
19 25/35
25/35 can reduce to 5/7
answer = 19 5/7