-4
$\begingroup$

Two even numbers are selected from the first ten even numbers not including zero. What's the probability that the sum of these two numbers is 30.

My instructor said 3/10. Is this really correct?

$\endgroup$
  • 9
    $\begingroup$ Selected in what way? With equal probability? With replacement or without replacement? Did you try listing out the possible pairs of numbers? $\endgroup$ – Glen_b -Reinstate Monica Oct 23 '17 at 23:14
1
$\begingroup$

There are 6 ways to select a tuple such that it adds up to 30 and overall 10*9 tuples (without replacement, 10*10 with). Therefore the asked probabilty should be 6/90 = 1/15. This is assuming that sample (i, j) is different from (i, j) (otherwise 45 options in total and only 3 tuples with the desired property -> gives again P=1/15)

$\endgroup$
  • $\begingroup$ For sampling with replacement,. why do you divide by 90 rather than 100 (= 10*10)? $\endgroup$ – Mark L. Stone Oct 24 '17 at 11:59
  • $\begingroup$ As I wrote: for sampling with replacement you should use 10*10. Excuse me if that was formulated not clear enough. $\endgroup$ – Jojo Oct 24 '17 at 12:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.