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4)

a) - 3/5 + 4/15

= (- 3 x 3) / (5 x 3) + 4/15

= - 9/15 + 4/15

= - 5/15

= (5 x - 1) / (5 x 3)

= - 1/3

b) 3/4 + 5/- 6

= 3/4 - 5/6

= (3 x 3) / (4 x 3) - (5 x 2) / (6 x 2)

= 9/12 - 10/12

= - 1/12

c) - 2/3 - - 3/4

= - 2/3 + 3/4

= (- 2 x 4) / (3 x 4) + (3 x 3) / (4 x 3)

= - 8/12 + 9/12

= 1/12

d) 2/8 + - 1/- 16

= 2/8 + 1/16

= (2 x 2) / (8 x 2) + 1/16

= 4/16 + 1/16

= 5/16

e) 5/2 - - 9/10

= 5/2 + 9/10

= (5 x 5) / (2 x 5) + 9/10

= 25/10 + 9/10

= 34/10

= (2 x 17) / (2 x 5)

= 17/5

f) 1/3 + 12/21

= (1 x 7) / (3 x 7) + 12/21

= 7/21 + 12/21

= 19/21