Question 236927
I am assuming that the probability of rain is 90% PER DAY and that the question is what is the probability that it will rain on any 2 days EXACTLY, not more or less.
Prob=.9=x, Prob no rain=.1=y
we need to take the x+y binomial to the 10th power
(x+y)^10 the 2nd from the last term is x^2y^8
nCr=10C2=45 the coefficient in front of x^2y^8 
So here's the formula:
prob={{{45*.9^2*.1^8=3.645*10^(-7)}}}
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Ed