# Stats homework -- binomial distribution help?!?

this is the question that I need to answer for an assignment :
Maryanne is a volunteer with the Canadian Red Cross. There are 4 blood types: O, A, B, or AB. Each blood type can also be Rhesus positive (Rh+) or Rhesus negative (Rh-). The probability of being Rh+ is 0.85. Maryanne helps 7 individuals during her...
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this is the question that I need to answer for an assignment :

Maryanne is a volunteer with the Canadian Red Cross. There are 4 blood types: O, A, B, or AB. Each blood type can also be Rhesus positive (Rh+) or Rhesus negative (Rh-). The probability of being Rh+ is 0.85. Maryanne helps 7 individuals during her shift and none of them are related to any of the others. Find the probability that:

a) Exactly 5 people are Rh+.

b) Less than 3 people are Rh+.

c) 3 or more people are Rh+.

d) Exactly 0 people are Rh-.

I know that the formula I need to use for this is P(X=x) = nCrP^xq^n-x where p=probability of Rh+ or 0.85, q=probability of Rh- or 1-0.85 which = 0.15, n = 7 and x changes for each question. The problem that I am having is that I dont know how to work this formula.. is there a certain way to do order of operations when Combination (nCr) is a part of the formula? all my answers keep comming out as above 1, when I know that they should be less than one, so I know that I must be doing something wrong. please help!

Maryanne is a volunteer with the Canadian Red Cross. There are 4 blood types: O, A, B, or AB. Each blood type can also be Rhesus positive (Rh+) or Rhesus negative (Rh-). The probability of being Rh+ is 0.85. Maryanne helps 7 individuals during her shift and none of them are related to any of the others. Find the probability that:

a) Exactly 5 people are Rh+.

b) Less than 3 people are Rh+.

c) 3 or more people are Rh+.

d) Exactly 0 people are Rh-.

I know that the formula I need to use for this is P(X=x) = nCrP^xq^n-x where p=probability of Rh+ or 0.85, q=probability of Rh- or 1-0.85 which = 0.15, n = 7 and x changes for each question. The problem that I am having is that I dont know how to work this formula.. is there a certain way to do order of operations when Combination (nCr) is a part of the formula? all my answers keep comming out as above 1, when I know that they should be less than one, so I know that I must be doing something wrong. please help!

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