Answers

2014-11-05T18:29:17+08:00
Multiples:
15:  15, 30, 45, 60, 75, 90 . . .
10: 10, 20, 30, 40, 50, 60, 70, 80, 90 . . .
5: 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90 . . .
common multiples for the 3 numbers:  30,60,90
least common multiple: 30

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2014-11-07T08:11:59+08:00
You have to list possible multiples of all then find those who matched

for 15 -> 15 30 45 60 75 etc
for 10 -> 10 20 30 40 50 etc
for 5 -> 5 10 15 20 25 30 etc

We have 30, but there are more common multiples aside from it if you just continue listing

like 60 90 120 150 etc :)
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