Your brother is 1/3 your age your sister is 6 years older then your brother your sister is 10 years old write and solve an equat
2 answers:
You might be interested in
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