how many 4 digit numbers...

[ket]

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

a. 16
b. 80
c. 800
d. 1440
e. 1600

I dont have OA.

My approach: the first number must be prime i.e one from 2,3,5,7 and therefore there are 4 ways to chose it. the second number can be any digit i.e. any from 1,2,3,4,5,6,7,8,9 therefore - 9 ways to chose, it the same goes to third number. as for fourth number, that also needs to be a prime there are again only 4 ways to chose it

All in all I get 4X9X9X4 = 1296 ... but there is no answer as such.. I must be doing it completely wrong

Please help.

thanks

[mike22629]

First of all, I have seen many questions like this on the internet and posted on the forum and just to let you know, these types of questions are not usually on the GMAT.

But...

4*4*10*10 = 16

You forget that the possible numbers for the two middle digits can also be 0.

[mike22629]

Forgot to write the last two zeros.

Answer 1600

[ket]

Thanks A LOT mike22629 .