Estimated difference of 7 17/20 - 6 2/5

Is always easier to subtract or add fractions with the same denominator, so
6 2/5 is the same as 6  8/20 
I would suggest turning them both to improper fractions, 
(You multiply 6 by the denominator 20 and get 120 then add 8)
6 8/20 = 128/20 
and then do the same with 7 17/20  (Mutiply 7 and 20 and then add 17)
7 17/20 = 157/20

157/20 - 128/20 = 29/20 or 1 9/20