My Math Forum Binomial Distribution Probability

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 October 27th, 2017, 06:33 AM #1 Newbie   Joined: Oct 2017 From: Texas Posts: 1 Thanks: 0 Binomial Distribution Probability A toy manufacturing company tests the quality of the toys it manufactures. On a daily basis, 20 toys are taken at random for testing and to check that 95% meet the toy specification. The day's production will be accepted provided that no more than 2 toys fail to meet the specification standard. Calculate the probability if the inspectors pass the day's toy production as acceptable if 75% of the day's toys meet the specification. n=20 Success(P)=0.75 Failure(Q)=0.25 =(^20 C 19)(0.75)^19(0.25)^20−19 =0.0211 The result seems lower than what I would expect. Where have I gone wrong? Thanks.
 October 27th, 2017, 07:27 AM #2 Newbie   Joined: Oct 2017 From: US Posts: 13 Thanks: 1 "No more than 2" does not mean "1". Instead, it means "0, 1 and 2". Therefore, it should be (^20 C18 )(0.75)^18(0.25)^20−18 + (^20 C19 )(0.75)^19(0.25)^20−19 + (^20 C20 )(0.75)^20(0.25)^20−20 = 0.06694781+0.02114141+0.003171212=0.09126043 To make it easier, use this free calculator online by inputting (2/20/0.25), and you will automatically get the cumulative probability of 0.09126043 for no more than 2 failures. Here, as the target is to calculate failure, we use 0.25 instead of 0.75.

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