what is the probability you will see a full moon?

Question

A full moon occurs every 29 nights. If you choose a night at random and then observe the night sky for 11 successive nights, what is the probability you will see a full moon?

1.   0.03
2.   0.11
3.   0.38

Comments

oh,so the question asks for any night among 11 succesive nights??

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Yes, it can be any night.

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thanks @kapil

Answer 1

1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
16	17	18	19	20	21	22	23	24	25	26	27	28	29	1

If there is a full moon on 29th then total possible start nights are -

1. If we observe 11 successive nights starting from 18th then on 11th night we will get a full moon.

Similarly,  we can start from 19th , 20th, ......, 29th.

But If we start on 1st ( or 2nd to 17th) night then we will be observing 11 successive nights from 1, 2, 3, 4, ....to 10th night but we won't get full moon because 29th night( full moon night as per assumption) is not coming in those 11 successive nights.  

So, Probability = 11/29

                       = 0.38 

Comments

I didn't get the point.When you are choosing 1st as a night and then counting 11 successive nights you are starting from 1 only i.e 1,2,3...........10th. But when you are selecting 18th then you are starting from 19th i.e. 19,20,21,22,23,24,25,26,27,28,29.

Please explain.