Equations:
m = k + 5
k = s + 2
m + k + s = 30
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Substitute for "m" and for "s"::
k+5 + k + k-2 = 30
3k +3 = 30
3k = 27
Kirsty has 9 marbles
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Mina has 9+5 = 14 marbles
Seb has 9-2 = 7 marbles
Divide 120 by 4 to get 1/9 of the rope: 30. Then multiply it by 9 to get the part after 1/4 was given away: 270. Now divide it by 3 to get 90 and lastly, multiply it by 4 to get a final answer of 360 centimeters. Hope this helps!
