A number is selected at random from 18-46 (inclusive)....

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A number is selected at random from the range 18-46(inclusive). What is the probability that the number is either prime or only has two prime divisors?

There are 28 possible numbers that can be selected at random. 7 of which I found to be prime and 7 with only two prime divisors so the probability is 14/28. That was my answer. I got marked wrong. Can anyone help with this?
 
Duplicate post deleted.

Which numbers did you determine to be prime? Which did you determine to have two prime divisors? Are you sure you weren't supposed to convert your answer to lowest form and/or a percentage?

Thank you.

Eliz.
 
Which numbers did you determine to be prime?
19,23,29,31,37,41,43


Which did you determine to have two prime divisors?

21,25,33,34,35,38,39

It just dawned on me if the number has only 2 prime divisors, doesn't it have to be prime?

Are you sure you weren't supposed to convert your answer to lowest form and/or a percentage?
No, the answer is to be in fraction form and not reduced.
Thanks!
 
Does 5*5=25 have TWO prime factors or just one (that is used twice)?
 
Re: A number is selected at random from 18-46 (inclusive)...

3thestral3 said:
A number is selected at random from the range 18-46(inclusive)........
There are 28 possible numbers that can be selected at random.
Click to expand...
NO! There are 29 numbers, due to "inclusive".
Like, range 3 to 5 inclusive = 3,4,5
 
Re: A number is selected at random from 18-46 (inclusive)...

3thestral3 said:
A number is selected at random from the range 18-46(inclusive). What is the probability that the number is either prime or only has two prime divisors?
Click to expand...
This is the list that fits the criterion: 19, 23, 29, 31, 37, 41, 43, 21, 22, 26, 33, 34, 38, 39, 46.
Note that 25 is not included because it has only one prime divisor.
 
Wouldn't 18 fit on the list, since 18 = (2)(3)(3), and thus has two prime divisors (one occurring twice)?

Eliz.
 
stapel said:
Wouldn't 18 fit on the list, since 18 = (2)(3)(3), and thus has two prime divisors (one occurring twice)? Eliz.
Click to expand...
GOOD CATCH Eliz.
I think we can all agree this is poorly put problem.
I suspect that whoever wrote this problem meant for 25 to also include in the list.
Then if we include 18 we must include 45=(5)(3)(3).
I of course was reading it as “two and only two factors both of which are prime”.
But clearly that is not what was said.
 
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