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2013-09-13T17:22:18+02:00
A= 1-1/3-2/20
A = (20x3-20-6)/60
A = (60-26)/60
A= 34/60 = 17/30

B = (1-1/3-1/4)x2/5
B = ((12-4-3)/12)x2/5
B = 5/12 x 2/5
B = 5x2/12x5 = 10/60 = 1/6