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a / 30 = 10+a / 42
42a = 300 + 30a
42a - 30a = 300
12a = 300 
a = 300 : 12
a = 25 cm

b/15 = 4,5 + b / 21
21b = 67,5 + 15b
21b - 15b = 67,5
6b = 67,5
b = 67,5 : 6
b = 11,25 cm

c / 8 = 10,5 / 14
c = 10,5/14 x 8
c = 84 / 14
c = 6 cm

d/12 = d+6 / 20
20d = 12d + 72
20d - 12d = 72
8d = 72
d = 72 : 8
d = 9 cm