LCM and what else?

[kwame.appau]

am confused with this question
-five bells commence tolling together and tolls at an intervals of 2,5,6,8 and 9 seconds respectively. how many times do they toll together in an hour?
source: pocket math GRE

[tommywallach]

Hey Kwame,

This is just an LCM question. What's the LCM of 2, 5, 6, 8, 9?

We take the largest number of each available prime factor. 8 gives us three 2s, 9 gives us two 3s, and the 5 gives us 5!

2 * 2 * 2 * 5 * 3 * 3 = 360

So the bells meet every 360 seconds. If one hour is 3600 seconds, they should toll together 10 times. Make sense?

-t

[kwame.appau]

Hi Tommy!
what about the time the clocks started together? did they started tolling together at the 360th seconds or the 0th second.
Thanks

[tommywallach]

Hey Kwame,

Oops! You're right! The question says they commence by tolling together, you're right that we would have to count that one as well. So the answer is 11. Sorry!

-t