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I am doing some revision for an exam I have tomorrow and I can't work out how an answer is achieved.

The weight $X$ kg of a bag of cement can be modelled by a normal distribution with mean $50$ kg and standard deviation $2$ kg.
$P(x > 53) = 0.0668$

Find the probability, $a$, that two weigh more than $53$ kg and one weighs less than $53$ kg.

So I did: $a = 0.0688^2 \times (1-0.0688)$

However the answer is $a = 3 \times 0.0688^2 \times (1-0.0688)$

I can't understand where the multiplication by $3$ comes from, although three are three bags, surely that has already been taken into account by multiplying $3$ probablities?

(The exam is tomorrow so I would appreciate answers before then, but I would still like to know the answer irrespective of the exam)

  • 0
    In tex mode, use `\times` instead of `*` i.e. $\times$ instead of $*$2012-05-17

3 Answers 3