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Permutation/Combinations question

by jsl » Mon Oct 27, 2008 3:03 pm
L1 and L2 are two parallel lines in a plane. If L1 has 6 points equally spaced along its length and if L2 has 5 points also equally spaced along its length, how many different triangles can be formed by connecting the points on the two parallel lines?

135
90
60
30
15
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by earth@work » Mon Oct 27, 2008 3:12 pm
IMO A
Taking line L1, total triangles= 6x5C2 = 60 ways
Taking line L2, total triangles = 5x6C2 = 75 ways
Total = 60 +75 = 135 ways
what is the OA
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by stop@800 » Tue Oct 28, 2008 2:19 am
yup, has to be A
6c2 * 5c1 + 6c1 * 5c2

135
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by jsl » Tue Oct 28, 2008 2:59 am
OA: A

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