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
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
