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Too Old For Grandma (Posted on 2003-02-28) Difficulty: 2 of 5
"I've always been 45 years older than your Dad", said Grandma to young grandson.

"But now the two digits in my age, both prime, are the reverse of the two in your Dad's age".

How old is Grandma?

See The Solution Submitted by Ravi Raja    
Rating: 2.5000 (14 votes)

Comments: ( Back to comment list | You must be logged in to post comments.)
Solution Puzzle Solution With Explanation | Comment 15 of 16 |
(In reply to answer by K Sengupta)

Let the age of the grandma be 10R + S.

Then, by the given conditions, the age of Dad is 10S+R

Accordingly, we must have:

(10R+S) - (10S+R) = 45
or, 9(R-S) = 45
or, R-S = 45/9 = 5

We know that each of  R and S is a  one digit prime number.

Since, the difference between them is odd, it follows that S must correspond to the only even prime number, that is 2.

This gives R = 5+ S = 5+2 = 7

Consequently, Grandma is precisely 72 years old. 

Edited on March 12, 2008, 7:18 am
  Posted by K Sengupta on 2008-03-12 06:47:05

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