If beaver family one can cut down and 20" tree in 15 minutes and family two can cut down a tree 3 times the size in twice as lon
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Answer:
Step-by-step explanation:
Midpoint formula is
---------------(1)
Here
Substituting values in equation (1)
A random variable following a binomial distribution over 
trials with success probability 
has PMF
Because it's a proper probability distribution, you know that the sum of all the probabilities over the distribution's support must be 1, i.e.
The mean is given by the expected value of the distribution,
The remaining sum has a summand which is the PMF of yet another binomial distribution with
trials and the same success probability, so the sum is 1 and you're left with
You can similarly derive the variance by computing
, but I'll leave that as an exercise for you. You would find that 
, so the variance here would be
The standard deviation is just the square root of the variance, which is
Because it's a proper probability distribution, you know that the sum of all the probabilities over the distribution's support must be 1, i.e.
The mean is given by the expected value of the distribution,
The remaining sum has a summand which is the PMF of yet another binomial distribution with
You can similarly derive the variance by computing
The standard deviation is just the square root of the variance, which is