Question

20 hair pens are bad in a group of 144 ball pens. And the rest are good. You will want to buy the pen
that is good but you will not want to buy the bad pen. The shopkeeper takes out one of these pens at random and gives it to you , what is the probability that 
( i ) You will buy that pen ( ii ) You will not buy​

Answer 1

Step-by-step explanation:

Total number of ball pens = 144

Defective ball pens = 20

Ball pens in good condition = 144−20=124

Let A = An event such that you will buy good ball pen

∴n(A)=124

P(A)=n(S)n(A)

∴P(A)=144124=3631

2) Let A = Complementary event of event A

∴A = An event such that you will buy defective ball pen

∴n(A)=20

P(A)=n(S)n(A)

∴P(A)=14420=365

Answer 2

Step-by-step explanation:

Total number of ball pens = 144

Defective ball pens = 20

Ball pens in good condition = 144−20=124

Let A = An event such that you will buy good ball pen

∴n(A)=124

P(A)= n(A)/n(S)

∴P(A)= 124/144 = 31/36

2) Let

A

= Complementary event of event A

∴

A

= An event such that you will buy defective ball pen

∴n( A )=20

P(A) = n(a) /n(S) = 20/144

= 36/5