(20-а) - 6 7/18 = 3 17/18
20- а = 3 17/18 + 6 7/18
20 -а = 10 6/18
а=20 - 10 6/18
а= 9 12/18
а= 9 2/3
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(20 - 9 2/3 ) - 6 7/18 = 3 17/18
(20 - 9 12/18) - 6 7/18 = 3 17/18
10 6/18 - 6 7/18 = 3 17/18
186/18 - 115/18 = 3 17/18
71/18 = 3 17/18
3 17/18 = 3 17/18