[ArcySparky]

ARGH!

So simple...


The fact that it was a green threw me off... if it was a red or orange I'd have solved it like that!

as it was I was using all the knowledge gained from A-level statistics to try and solve it.

All well...

~Arcnowknowsnottooverthink...

[Rand0m]

I thought the answer was going to be

Spoiler (Rollover to View):

forever

because of the heavily stressed randoness - I guess the reason it isn't is because of the bit that says on average - ie

Spoiler (Rollover to View):

some people will see it first time, others will wait until Doomsday, but on average most people will be certain of having seen it by 140.

I think.

[Tufty]

What I dont understand is that as it's completely random, i.e. as soon as it switches from one picture to the next it "forgets" which pictures have been shown, this fellow's portrait might never come up.

And also, if it was all 140 uniquely then on average then it would be 70, as out of 140 shots half the time he'd be in the first 70 and so on.

Hmmm.

[TopGun2]

You're overcomplicating this card, it's only a green.

Spoiler (Rollover to View):

On average the portrait will appear once every 140 minutes

Spoiler (Rollover to View):

Personally I'd knock 1 minute off for the fact that one portrait is shown when he arrives but the answer doesn't do that

so the confirmed solve is:

Spoiler (Rollover to View):

140

[thereverendeg]

It doesn't accept any of these:

Spoiler (Rollover to View):

96 97 98

[KingOfWrong]

As fitzyfitz noted:

Spoiler (Rollover to View):

The probability of waiting k minutes, P(X=k) = p(1-p) k-1 where p is the chance of your picture appearing: 1/140

This distribution is:

Spoiler (Rollover to View):

X ~ Geometric(p)

[fitzyfitz]

Spoiler (Rollover to View):

probability of observer *not* seeing their portrait after X minutes is 139/140 ^ X (139/140)^96 = 0.502 (139/140)^97 = 0.499 so it takes 97 minutes to reach a >50% chance of seeing your own portrait

[norman182]

#128 Perplexing Portraits