Math destroyer problem

[ekephant]

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A recent survey determined that two-fifths of practicing dentists use a water pik on their own teeth. If 10 dentists are chosen at random, which expression represents the probability that exactly three of them use a water pik on their own teeth?

The answer is C(10,3)•(2/5)^3•(3/5)^7

Can someone please explain what this means? I don't remember what the C even means. Thanks in advance!

 

[Okilito]

This is an equation for binomial probability. C stands for combination formula where n = 10 (dentists) and r= 3 (dentists that can be chosen) it is multiplied by (2/5)^3 because probability that one picked dentis uses a water pic is 2/5 and we are trying to cacluate the probability for 3 dentists. The last term is the probability of dentists who do not use water pic (1 - 2/5 = 3/5) and since the remaing detists are 10-3 = 7 it should be to the power of 7. Hope that helps.
You can find the actual formula at the begining of MATH destroyer.