# Another very easy one...

HI
My turn to twist ur brains..Answer this simple question..

You and 2 other people have numbers written on your foreheads.
• You are all told that the numbers on your foreheads are prime and that they form a triangle with a prime perimeter.
• You see 5 and 7 on the other 2 people.
• All three of you claim you do not know the number on your forehead, starting from you and now again it is your turn.

## Replies

• durga ch
in a triangle sum of 2 sides is greater third side

so , if x is the unknown number
7+5>x;
x+5>7;
x+7>5;

this leads us to result 12>x>2;
the intermediate primes are 3,11, 7,5
can two people have same numbers?
if not then my forehead has either 11 or 3 wirtten on it..

so if x=3 then sum is 3+5+7=15 which is not a prime number ( discarded)
if x=11 sum = 7+5+11 =23 is a aprime number

so.. the number on my head is 11 ( if any only if all three of us have different numbers written on our forehead)
• silverscorpion
Hi durga,
Sorry I didnt mention this, but two people can have the same number on their foreheads. Now how does this change your answer??
• durga ch
oh in that case...
assuming x=5
5+5+7=17 which is also prime
if x=7
7+7+5 is 19 which as well is prime...

so x can take any of 5 ,7 ,11
• silverscorpion
no, x cannot take any value. There's only one value x can take..
U r on the right track, try a bit more...
• electron1212
Hi Guys,
I am also agree with Durga. Reason being, after considering all the constraints there are 3 possible solutions i.e. 5, 7 and 11. All three of these satisfying the condition so no way of discarding any 2 values, Untill and unless number written on foreheads are unique and if that is the case 11 is the desired number.

You are reading an archived discussion.

## PWM frequency

I plan on getting a motor and controlling it with pulse width modulation. I noticed some motors don't function that well when the frequency of the PWM is too high...

## Final year mechanical project

hello...... i m final year student of branch mechanical.......suggest me the topic for project. which is cheep and best .plz i canot think about my topic.plz help me.

## Greatings CE forum.

Name: Lock-Os *Engineering Trade: Mechanical Location: Pittsburgh, PA *Occupation: Student at Penn State University Work Experience: None *Hobbies & Interests: Scale Models, Blacksmithing, Anime, Military History, Taking things apart &...

## iCBBE2009 Call for Paper(Ei Index) LF

========================================================================================= The 3rd International Conference on Bioinformatics and Biomedical Engineering (iCBBE2009) Call for Papers Beijing, China June 11-13, 2009 ======================================================================================== Within iCBBE 2009, there are two special sessions: Special...

## Difference between cp and mv commands in UNIX

Can someone explain the difference between "cp" and "mv" commands in UNIX?