globalcitizen wrote:So I got the wrong answer for this....
Can someone please explain to me why the calculation shouldn't be:
(1/10) - because there is only one 0
(4/10) - because there are only four prime numbers (2,3,5,7)
and thus,
(1/10)(4/10)(4/10)(4/10)(4/10)
When I look at the correct answers, I understand why 5/10 was used...and though I now know that my calculation is wrong, I still don't understand why it is wrong to breakdown prime number probability vs. likelihood of picking 0. If someone can be so kind as to explain why, I would be most appreciative. I.e. What does my incorrect calculation mean in words?
Thanks!
Let me try to explain. The first step of your approach is incorrect.
The problem asks to have either prime numbers or zero as password.
So, between 0 to 9, which digits will satisfy the above condition?
{0,2,3,5,7} - let me call this as PWD set
So we have 5 digits from which should appear in the password.
Now we have 5 digit password.
probability that first digit comes form PWD set is 5/10 [because we can pick any from PWD set out of the whole 10 digits available]
Similar is the case for other 4 digits of the password also.
Also, important thing to remember here is that the question is not asking for 5 digit numbers. So 0 can be in any position.
Hope this helps. Let me know in case you have any doubt.
