GATE | GATE-CS-2015 (Set 1) | Question 10

The probabilities that a student passes in Mathematics, Physics and Chemistry are m, p and c respectively. Of these subjects, the student has 75% chance of passing in at least one, a 50% chance of passing in at least two and a 40% chance of passing in exactly two. Following relations are drawn in m, p and c:

1. p + m + c = 27/20 
2. p + m + c = 13/20
3. (p) × (m) × (c) = 1/10 

(A) Only relation 1 is true
(B) Only relation 2 is true
(C) Relations 2 and 3 are true
(D) Relations 1 and 3 are true


Answer: (D)

Explanation:

1 - (1 - m) (1 - p) (1 - c) = 0.75               -------(1)
(1 - m)pc +  (1 - p)mc + (1 - c)mp + mpc = 0.5   -------(2)
(1 - m)pc +  (1 - p)mc + (1 - c)mp  = 0.4        -------(3)

From last 2 equations, we can derive mpc = 0.1

After simplifying equation 1, we get.
p + c + m - (mp + mc + pc) + mpc = 0.75        
p + c + m - (mp + mc + pc) = 0.65         -------(4)

After simplifying equation 3, we get
pc + mc + mp - 3mpc = 0.4

Putting value of mpc, we get
pc + mc + mp = 0.7

After putting above value in equation 4, we get
 p + c + m - 0.7 = 0.65
 p + c + m = 1.35 = 27/20 


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