Probability of Divisibility. (1 Viewer)

deswa1

Well-Known Member
Joined
Jul 12, 2011
Messages
2,256
Gender
Male
HSC
2012
Is it 2/3?
 
Last edited:

deswa1

Well-Known Member
Joined
Jul 12, 2011
Messages
2,256
Gender
Male
HSC
2012
Crap lol- I got all that (except for the digital root- not sure what that means) but I ended up with 2/3 (originally typo in previous post but I edited it...). Where did I make the mistake? (I'll try and post my working- give me a sec)
 
Last edited:

Fus Ro Dah

Member
Joined
Dec 16, 2011
Messages
248
Gender
Male
HSC
2013
Crap lol- I got all that (except for the digital root- not sure what that means) but I ended up with 2/3 (soz- typo in previous post). Where did I make the mistake? (I'll try and post my working- give me a sec)
I made a mistake because I miscounted the sequence. The answer is 2/3.
 

deswa1

Well-Known Member
Joined
Jul 12, 2011
Messages
2,256
Gender
Male
HSC
2012
I made a mistake because I miscounted the sequence. The answer is 2/3.
Oh ok- cool. That was a nice question- thanks Realise :)

And I didn't know that the sum of cubes was the triangular numbers squared- I wrote out the first ten terms of the cubes and noticed that haha so I learnt something new as well..
 

seanieg89

Well-Known Member
Joined
Aug 8, 2006
Messages
2,662
Gender
Male
HSC
2007
Pretty sure the answer is 2/3. The question is equivalent to finding the probability that n(n+1) is divisible by 3. This occurs precisely when n is congruent to 0 or 2 mod 3.

EDIT. You noticed this already, cool.
 

Users Who Are Viewing This Thread (Users: 0, Guests: 1)

Top