Calculus and Above
Calculus Questions? Ask a Mathematician for Answers ASAP
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If this is the problem that I think it is, the z-value that you should get is z = -0.2572, rather than z = +0.2572.
Then, since you want the probability of "at least one disaster in 25 missions", you want the area that is to the right of z = -0.2572.
You can use Excel to calculate this by using the following formula:
= 1 - normsdist(-0.2572)
which will give you 0.6015
Or, you can use a table to find the area to the right of z = +0.2572, and subtract that value from 1 to get 0.6015.
Please feel free to ask if you have any additional questions about this.