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Place four pebbles on the sand in the form of a square. Keep adding as few pebbles as necessary to double the area. How many extra pebbles are added each time?

Many Clues, One Answer

Stage: 3 Short Challenge Level: Challenge Level:1

Find the number:
 

it's less than $100$;
it's one more than a multiple of $3$;
exactly one of the digits is prime;
if you reverse its digits, you get a prime;
it has exactly four factors;
the sum of its digits is prime;
if you multiply it by $5$, the answer is greater than $100$.
 
 
If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas. 

 

 

This problem is taken from the UKMT Mathematical Challenges.

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