A prime number such as 379 is called a 'super prime' because any integ

Question

A prime number such as 379 is called a 'super prime' because any integers obtained by deleting digits from the right of 379 are prime; for example, 37 and 3 are both prime. How many two-digit positive superprime numbers exist?
A) 9
B) 10
C) 11
D) 12
E) 13

chetan2u · Answer

This means we have to look for all prime numbers with tens digit as 2, 3, 5 or 7.

1) 2 : 23, 29
2) 3 : 31, 37
3) 5 : 53, 59
4) 7 : 71, 73, 79

Total = 2+2+2+3 = 9

A

noobieTopro · Answer

(1)Tens digit should be prime. 
(2)Overall two digit number should be prime.

1) 23, 29 = 2 
2) 31, 37 = 2 
3) 53, 59 = 2 
4)71, 73, 79 = 3

Total = 9
Therefore, answer is A.

Kapil1650 · Answer

This is easy to ascertain if asked for two digit number what is is is asked for a three digit number ? Do we have any shortcut for it?

Posted from my mobile device

KarishmaB · Answer

You are expected to know all the prime numbers from 1 to 100. It does come in very handy. But three digits would be too cumbersome.

First and second must be 23/29/31/37/53/59/71/73/79

Now you need to check for 
231/233/237/239
291/293/297/299
311/313/317/319
... and so on.

We need to have some more constraints to narrow down the scope.

TarunKumar1234 · Answer

How many two-digit positive superprime numbers exist?

between 20 and 30- 23 and 29.
between 30 and 40- 31 and 37.
between 50 and 60- 53 and 59.
between 70 and 80- 71,73 and 79

So, Total 9 nos. I think A

TheUltimateWinner · Answer

Thanks for the explanation  VeritasKarishma 
It's all about counting the prime number. Is the any other way to solve this question without counting the prime numbers?
thanks..

KarishmaB · Answer

Prime numbers do not exist in any logical order. You cannot say which next number will be prime. There are just some tips that help us such as "they must end in 1/3/7/9", "they must be of the form 6a+1 or 6a-1."
Hence, you memorise what you can and the rest you will need to calculate if it comes to that. GMAT will not give you anything cumbersome.

bumpbot · Answer

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A prime number such as 379 is called a 'super prime' because any integ

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