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a = 1/15-7/6 = 2/30 -35/30 = -33/30 = -11/10

b = 7/12 -13/18 =21/36 -26/36 = -5/36

c =(-7/15+20/15) -(-21/15 +20/15) = (13/15) -(-1/15 )= 13/15+1/15 = 14/15

d = 63/21 -49/21 -2/21 = 63/21 -51/21 =12/21 =4/7

e = 3/7 x -28/27 = -   4 /7

f = 39/14 x - 21/78 = 1/14 x -21/2 = -21/28 = - 3/4

g =7/10-6/11 x 33/21 = 7/10 - 6 x 3/21= 7/10- 6x 1/7= 7/10 -6/7 = 49/70-60/70 = -11/70