IIT JEE , AIEEE Forum - Mathematics , Algebra

nehasharma07

Q The probability that a teacher will give an unannounced test during any class meeting is 1/5. If the student is absent twice, the probability that he will miss atleast one test is

a)7/25

b)9/25

c)16/25

d)24/25

LAMPARD

Its 9/25...

LAMPARD

probablity that he misses on one day only
(4/5)(1/5)*2
=8/25
prob that he misses both tests
(1/5)(1/5)
=1/25
total=9/25

actually converse is easier...that he doesnt miss even 1 test
=4/5*4/5=16/25
so P'=9/25

Shesha

ans is 9/5

probability that a test is conducted on any one of the days that the student is absent = 4/5*1/5 +1/5*4/5-----------(1)

probabability that a test is held on both the days = 1/5*1/5-------(2)

therefore probability that the student will miss ATLEAST one test isgiven from (1) and (2)....

= 4/5*1/5 +1/5*4/5 +1/5*1/5 = 9/5

Rate me if satisfactory....

nehasharma07

thanks

punnima

ans is 9/25
p(not missing a test)=(4/5)*(4/5)=16/25
p'=9/25

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