Probability -Aptitude

1.

Let A and B be independent events with P (A) = 0.7 and P(B) = 0.7. Find P(A n B)? 

   A.) 4.9
   B.) 0.049
   C.) 0.49
   D.) None of these

Answer: Option 'C'

P (A n B) = P(A) . P(B) 
P (A n B) = 0.7 . 0.7 
P (A n B) = 0.47.

2.

6 Coins are tossed simultaneously. find the probability to get 2 hands 

   A.) 15/32
   B.) 5/64
   C.) 15/64
   D.) None of these

Answer: Option 'C'

26 = 64, ( 1 = 2, 2 = 4, 4 = 16, 16 = 32, 32=64 times ) 
6c2 = 6!/2! = (6 × 5 × 4 × 3 × 2 × 1)/ (6 - 2)! × (2 × 1) = 15
Probability = 15/64.

3.

If A and B are two events such that P (A) = 3/4, P (B) = 1/2 and P (A n B) = 3/8, find P (not A and not B). 

   A.) 3/8
   B.) 1/8
   C.) 1/4
   D.) None of these

Answer: Option 'B'

P(not A and not B) = 1 - (P(A) + P(B) - P(AB)) 
which you might find somewhere in your text.
Substituting in our probabilities we get: 
P(not A and not B) = 1 - (3/4 + 1/2 - 3/8) 
P(not A and not B) = 1 - (7/8) 
P(not A and not B) = 1/8.

4.

An unbiased die is thrown twice. Let the event A be ‘odd number on the first throw’ and B the event  ‘odd number on the second throw’. Check the independence of the events A and B. 

   A.) A or B are independent events
   B.) A and B are not independent events
   C.) A and B are independent events
   D.) None of these

Answer: Option 'C'

If all the 36 elementary events of the experiment are considered to be equally likely, we have
P(A) = 18/36 = 1/2 
= and P(B) = 18/36 = 1/2 
Also P(A n B) = P (odd number on both throws)
= 9/36 = 1/4 
Now P(A) P(B) = 1/2 × 1/2 = 1/4 
P(A n B) = P(A) × P(B) 
A and B are independent events

5.

If P (A) = 0.18, P (B) = 0.5 and P (B|A) = 0.2, find P(A n B)? 

   A.) 0.32
   B.) 0.36
   C.) 0.16
   D.) 0.64

Answer: Option 'B'

P(B|A) = P(A n B)/P(A) 
P(A n B) = P(B|A) × P(A) 
P(A n B) = 0.2 × 0.18 
P(A n B) = 0.36

6.

If P(A) = 2/15, P(B) = 4/15, and P(A ∪ B) = 6/15 Find P(A|B) 

   A.) 6/15
   B.) 3/4
   C.) 3/2
   D.) None of these

Answer: Option 'C'

P(A|B) = P(A ∪ B)/P(B) 
P(A|B) = (6/15)/(4/15) = 3/2.

7.

Three cards are drawn successively, without replacement from a pack of 52 well shuffled cards. What is the probability that first two cards are queens and the third card drawn is an ace?

   A.) 2/5530
   B.) 3/5525
   C.) 2/5525
   D.) 4/5525

Answer: Option 'C'

Let Q denote the event that the card drawn is queen and A be the event that 
the card drawn is an ace. Clearly, we have to find P (QQA)
Now P(Q) = 4/52 
Also, P (Q|Q) is the probability of second queen with the condition that one queen has
already been drawn. Now there are three queen in (52 - 1) = 51 cards.
Therefore P(Q|Q) = 3/51 
P(A|QQ) is the probability of third drawn card to be an ace, with the condition 
that two queens have already been drawn. Now there are four aces in left 50 cards.
Therefore P(A|QQ) = 4/50 
By multiplication law of probability, we have 
P(QQA) = P(Q) P(Q|Q) P(A|QQ) 
= 4/52 × 3/51 × 4/50 
= 2/5525.

8.

An urn contains 10 black and 5 white balls. Two balls are drawn from the urn one after the other without replacement. What is the probability that both drawn balls are black? 

   A.) 1/7
   B.) 2/7
   C.) 7/3
   D.) 3/7

Answer: Option 'D'

Let E and F denote respectively the events that first and second ball drawn are black. We have to find P(E n F) or P (EF).
Now P(E) = P (black ball in first draw) = 10/15 
Also given that the first ball drawn is black, i.e., event E has occurred, now there are 9 black balls and five white balls left in the urn. Therefore, the probability that the second ball drawn is black, given that the ball in the first draw is black, is nothing but the conditional probability of F given that E has occurred.
That is P(F|E) = 9/14 
By multiplication rule of probability, we have 
P (E n F) = P(E) P(F|E) 
= 10/15 × 9/14 = 3/7

9.

A die is thrown three times. Events X and Y are defined as below:
X : 4 on the third throw 
Y : 6 on the first and 5 on the second throw 
What is the probability of X given that Y has already occurred. 

   A.) 2/6
   B.) 1/6
   C.) 1/5
   D.) None of these

Answer: Option 'B'

The sample space has 216 outcomes.
Now X = (1,1,4) (1,2,4) ... (1,6,4) (2,1,4) (2,2,4) ... (2,6,4)
(3,1,4) (3,2,4) ... (3,6,4) (4,1,4) (4,2,4) ...(4,6,4)
(5,1,4) (5,2,4) ... (5,6,4) (6,1,4) (6,2,4) ...(6,5,4) (6,6,4)
Y = {(6,5,1), (6,5,2), (6,5,3), (6,5,4), (6,5,5), (6,5,6)}
and X n Y = {(6,5,4)}.
Now P(Y) = 6/216 
and P (X n Y) = 1/216 

10.

What is the probability that when a hand of 6 cards is drawn from a well shuffled deck of 52 cards, it contains 2 Queen 

   A.) 291/1017926
   B.) 29187/1017926
   C.) 29187/101792
   D.) 2987/101926

Answer: Option 'B'

nCr = n!/(n-r)!r!
Total number of possible hands = 52C6
52C6 = (52!)/((52-6)! × 6!)
52C6 = 61075560.
Number of hands with 2 Queen and 4 non-Queen cards = 4C2 × 48C4
4C2 = (4!)/(2! × 2!) = 6.
48C4 = (48!)/(44! × 4!) = 3 × 47 × 46 × 45 = 291870
(other 2 cards must be chosen from the rest 48 cards)
P (2 Queen) = (4C2 × 48C4)/52C6 = 29187/1017926.

11.

If P(A) = 4/5 and P (B) = 2/5, find P (A n B) if A and B are independent events. 

   A.) 8/23
   B.) 8/25
   C.) 3/25
   D.) None of these

Answer: Option 'B'

P (A n B) = P(A) . P(B) 
P (A n B) = 4/5 . 2/5 
P (A n B) = 8/25.

12.

Given that E and F are events such that P(E) = 0.16, P(F) = 0.4 and P(E n F) = 0.4,
find P (E|F) and P(F|E) 

   A.) 3/4,2
   B.) 1/4,1
   C.) 1,1/4
   D.) None of these

Answer: Option 'C'

Here, E and F are events 
P(E|F) = P(EnF)/P(F) = 0.4/0.4 = 1 
P(F|E) = P(EnF)/P(E) = 0.4/0.16 = 1/4.

13.

A family has two children. find the probability that both the children are girls given that at least one of them is a girl? 

   A.) 1/4
   B.) 2/3
   C.) 1/3
   D.) 1/4

Answer: Option 'C'

Let b stand for boy and g for girl. The sample space of the experiment is 
S = {(g, g), (g, b), (b, g), (b, b)} 
Let E and F denote the following events : 
E : ‘both the children are girls’ 
F : ‘at least one of the child is a girl’ 
Then E = {(g,g)} and F = {(g,g), (g,b), (b,g)} 
Now E n F = {(g,g)} 
Thus P(F) = 3/4
and P (E n F )= 1/4 
Therefore P(E|F) = P(E ∩ F)/P(F) = (1/4)/(3/4) = 1/3

14.

Ten cards numbered 1 to 10 are placed in a box, mixed up thoroughly and then one card is drawn randomly. If it is known that the number on the drawn card is more than 3, what is the probability that it is an even number? 

   A.) 2/7
   B.) 6/7
   C.) 7/2
   D.) 4/7

Answer: Option 'D'

Let A be the event ‘the number on the card drawn is even’ and B be the 
event ‘the number on the card drawn is greater than 3’. We have to find P(A|B). 
Now, the sample space of the experiment is S = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10} 
Then A = {2, 4, 6, 8, 10}, B = {4, 5, 6, 7, 8, 9, 10} 
and A n B = {4, 6, 8, 10} 
Also P(A) = 5/2, P(B) = 7/10 and P(A n B) = 4/10 
Then P(A|B) = P(A n B)/P(B) = (4/10)/(7/10) = 4/7


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