Răspuns:
Explicație pas cu pas:
8.
Primul tractorist 6/15 + 5/15 = 11/15
Al doilea tractorist 3/17 + 9/17 = 12/17
9.
3/28 + a = 23/28
a = 23/28 - 3/28 = 20/28 = 5/7
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87/144 - b = 5/12
b = 87/144 - 5/12 = 87/144 - 60/144 = 27/144 = 3/16
_________
9/11 - 40/143 = 117/143 - 40/143 = 77/143 = 7/13