a, 5/7 . 5/11 + 5/7 . 6/11 - 2/7
= 5/7 . (5/11 + 6/11) - 2/7
= 5/7 . 1 - 2/7
= 3/7
b, 1/2x - 33/55 = 20/50
1/2x - 3/5 = 2/5
1/2x = 2/5 + 3/5
1/2x = 1
x = 1 : 1/2
x = 2
c, 5/6 + 1/5 - 2/3
= 25/30 + 6/30 - 20/30
= 11/30
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