How many 4 digit numbers begin with a digit that is prime : PS Archive
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# How many 4 digit numbers begin with a digit that is prime

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Senior Manager
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How many 4 digit numbers begin with a digit that is prime [#permalink]

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16 Oct 2006, 21:24
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100% (01:47) correct 0% (00:00) wrong based on 1 sessions

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How many 4 digit numbers begin with a digit that is prime and end with a digit that is prime

A)16
B)80
C)800
D)1440
E)1600
Manager
Joined: 30 Jun 2006
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16 Oct 2006, 23:08
E for me.

The units place can be filled ini 4 ways ( 2, 3 ,5,7)
The tenth and the hundreds place can each be filled in 10 ways ( 0,1,2 ... 9)
The thousands place can be filled ini 4 ways ( 2, 3 ,5,7)

Hence the number of required numbers are 4 * 10 * 10 * 4 = 1600

Hope this to be correct.
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17 Oct 2006, 02:15
E for me as well, same reasoning as jainvik7

what is the OA?
Senior Manager
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17 Oct 2006, 06:14
OA is E..
What about the numbers that are repeated, though?

like 2112 or 2222 and so on..?
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17 Oct 2006, 17:20
E

Possibilities for the first digit 4; last digit 4;the other two digits 10

4*4*10*10 = 1600
17 Oct 2006, 17:20
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