# looks easy but I keep getting irrelevant answer

Thanks ! It is estimated that 70% of motorists on a certain stretch of highway exceed the speed limit. A highway patrolman with a radar device randomly selects 15 motorists on this part of the highway and checks their speed. What is the probability that at least 13 are caught speeding? A. 0.036 B. 0.092 C. 0.128 D. 0.908

15Choose13 0.7^13 0.3^2 + 15Choose14 0.7^14 0.3^1 + 15Choose15 0.7^15 0.3^0 if I’m not mistaken.

It says “at least” so I think we should sum-up all 13 binomial probabilities, not the last three (like Dreary wrote). Not quite sure about this one. p.s. I tried using Excel spreadsheet on this one (not sure I know how) and a got 0.092. Milos

clearly: (15 ncr 13) * 0.7 ^13*0.3^2+ (15 ncr 14) * 0.7 ^14*0.3^1 (15 ncr 15) * 0.7 ^15*0.3^0

Dreary is right - “at least 13” means 13 + 14 + 15 answer is 12.8%

Jes, that’s the correct answer. Thanks for clearing that out. Milos

Thanks a lot ! 27 days left and that most of my score are in the 60 … Time to freak out

D’Artagnan Wrote: ------------------------------------------------------- > Thanks ! > > It is estimated that 70% of motorists on a certain > stretch of highway exceed the speed limit. A > highway patrolman with a radar device randomly > selects 15 motorists on this part of the highway > and checks their speed. What is the probability > that at least 13 are caught speeding? > > > A. 0.036 > B. 0.092 > C. 0.128 > D. 0.908 105*(.7^13*.3^2) +15*(.7^14*.3) + 1*(.7^15*.3^0) ~.127