Question

there are 30 days in november. it rains on 3/5 of them. Work out the number of days when it does not rain.​

Answer 1

Or think about it this way: we want the probability of the following event: RRRNN, where R represent rain day and N represents no-rain day. Now, each R and each N have individual probability of 1/2, so .

But the case of RRRNN can occur in many ways: RRRNN, RRNRRN, RNRRN, NRRRN, ... basically it will be equla to # of arrangements (permutations) of 5 letters RRRNN out of which there are 3 identical R's and 2 identical N's. That # of arrangements is , (notice that it's the same as ). So, finally .

Answer 2

Answer:

According to the question 3/5of the month rains

Days on which it doesn't rains = 2/5

2/5 of 30 =12 days