## anonymous 3 years ago Need fast help: 20 students are writing a test. Each of them has a 75% chance to succed. How high is the chance, that 17 students succed?

1. anonymous

*that at least 17 succed.

2. anonymous

probability that 17 students succeed is 85%

3. anonymous

lol are you *in* a test? x)

4. anonymous

but since each of their's chance is NOT 100% then .....??

5. anonymous

i think it's 1-20c3 * 0.25*0.25*0.25

6. anonymous

Nope, but I'm going to be in at least 2 hours :P

7. anonymous

haha good cramming ;P I was gonna try to answer and then decided I didn't want to make you fail x) Good luck though! oh and *succeed ;)

8. anonymous

So 1-20c3*0,25*0,25*0,25 = 16,8125 % ?

9. anonymous

oh.....i was 16,8125 ?!

10. anonymous

*it

11. anonymous

|dw:1340020008729:dw|

12. anonymous

oh i l tell u my reasoning......... each one has a 75% probalilty of success...... so for 17 to scuceed........u need to select 17 students.......... multiply it with probabilty of success......ie 0.75^17..........or............3 fails ie 1- 20c3* 0.25^3

13. anonymous

@satellite73 m i correct?

14. anonymous

this is going to take a bunch of computation "at least' 17 means 17 or 18 or 19 or 20 use binomial for each $$P(x=20)=(.75)^{20}$$ $p(x=19)=20\times .75^{19}\times .25$ $P(x=18)=\dbinom{20}{2}.75^{18}\times .25^2$ $P(x=17)=\dbinom{20}{3}.75^{17}\times .25^3$ add these up

15. anonymous

Ah, alright!

16. anonymous

this is what i get, but check the calculations http://www.wolframalpha.com/input/?i=.75^20%2B20*.75^19*.25%2B190*.75^18*.25^2%2B1140*.75^17*.25^3

17. anonymous

Yeah. Same! Thank you very much!

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