pops wrote:2 consecutive prime can be 2,3 or 3,5 or 5,7
number of numbers formed with 2,3 = 2^3 however, question is using exactly two consecutive primes so we have to remove the case 222 and 333 hence number of such numbers = 2^3-2 = 6
so, number of such numbers for 2,3..2,5 and 5,7 = 3*6 = 18
number of three digit number = 900
hence, answer is 18/900 = 1/50
none of the answer choice..
but i guess my method is correct !
Assuming this is the correct interpretation of the question:
correct.. we can us 2,3 or 3,5 or 5,7.
we need to use at least one from each pair, so our possible 3 digits are:
2,2,3
2,3,3
3,3,5
3,5,5
5,5,7
5,7,7
For each set, there are 3 different ways to arrange the number (e.g. 2,2,3 can be 223, 232 or 322),
So, we have 6*3 = 18 possible arrangements
and your calculation of 18/900 = 1/50 seems dead on.
So, we have to conclude that there's another "correct" interpretation of the question, solidifying my original thought that it's a very poorly worded question ("What's the probability of creating a 3 digit number?" Well, if I'm eating breakfast, the probability is very small!). Good news - you'd never see anything ambiguous on the actual GMAT.
What's the source of the question? In the future, please ALWAYS post the source, so we know whether we should take the question seriously or can just write it off.
