Triple trouble

Logic Level 2

Irene is one of a set of identical triplets. One of her sisters always tells the truth, and the other one always lies. Irene sometimes is truthful and sometimes isn't.

The triplets are dressed in identical outfits except for the ribbons in their hair. One girl has a blue ribbon, one a green ribbon, and one a red ribbon.

"Irene?" you call out, questioningly.

"That's me," says the girl wearing a blue ribbon.
"Yep, she's Irene," agrees the girl wearing a green ribbon.
"I'm definitely not Irene," says the girl wearing a red ribbon.

Which triplet is Irene?

You need more information to be sure Girl wearing green ribbon Girl wearing blue ribbon Girl wearing red ribbon

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Denton Young
Jan 1, 2017

If the girl wearing a blue ribbon were Irene, they'd all be telling the truth, which is impossible since one of her sisters always lies.

If the girl wearing a red ribbon were Irene, they'd all be lying, which is impossible since one of her sisters is always truthful.

So Irene must be the girl wearing a green ribbon (and is lying). Her sister wearing red ribbon is the truth-teller, her sister wearing the blue ribbon is lying.

How about red=truth blue and green lie?

Reynan Henry - 4 years, 5 months ago

Log in to reply

That's correct. Irene is wearing the green ribbon, so her sister wearing a red ribbon is telling the truth that she's not Irene, and her sister wearing a blue ribbon is lying about being Irene. Irene also happens to be lying today.

Denton Young - 4 years, 5 months ago

You have shown that "green ribbon is a necessary condition". For completeness (or to avoid the assumption that the question has an answer), you should show that the conditions can be achieved by assigning truth-teller / liar to red / blue accordingly.

Calvin Lin Staff - 4 years, 5 months ago

Log in to reply

Mr. Reynan Henry already did so. Red ribbon: truthful. Other two ribbons: liars.

Denton Young - 4 years, 5 months ago

Log in to reply

Can you place that in the solution itself, so that others do not need to scour the comments to obtain a complete solution? Thanks!

Calvin Lin Staff - 4 years, 5 months ago

Log in to reply

@Calvin Lin It is done.

Denton Young - 4 years, 5 months ago

Thanks for the solution!

Diana Bradish - 1 year, 2 months ago
Piotr Paweska
Feb 21, 2017

One simple way is to list the possibilities and just eliminate the impossible outcomes (where T-Always tells Truth, L-always lies, X-can either be lying or telling the truth.

blue green red OUTCOME Comments
T L X Impossible blue and green contradict each other
L T X Impossible blue and green contradict each other
T X L Impossible blue and red contradict each other
L X T OK no contradiction so green must be Irene
X L T Impossible green and red contradict each other
X T L Impossible green and red contradict each other

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...