1

JEE Main 2019 (Online) 10th January Morning Slot

An unbiased coin is tossed. If the outcome is a head then a pair of unbiased dice is rolled and the sum of the numbers obtained on them is noted. If the toss of the coin results in tail then a card from a well-shuffled pack of nine cards numbered 1, 2, 3, ……, 9 is randomly picked and the number on the card is noted. The probability that the noted number is either 7 or 8 is -
A
${{19} \over {36}}$
B
${{15} \over {72}}$
C
${{13} \over {36}}$
D
${{19} \over {72}}$

Explanation

$P\left( A \right) = {1 \over 2} \times {{11} \over {36}} + {1 \over 2} \times {2 \over 9} = {{19} \over {72}}$
2

JEE Main 2019 (Online) 10th January Evening Slot

If the probability of hitting a target by a shooter, in any shot, is ${1 \over 3}$, then the minimum number of independent shots at the target required by him so that the probability of hitting the target atleast once is greater than ${5 \over 6}$ is -
A
4
B
6
C
5
D
3

Explanation

$1 - {}^n{C_0}{\left( {{1 \over 3}} \right)^0}{\left( {{2 \over 3}} \right)^n} > {5 \over 6}$

${1 \over 6} > {\left( {{2 \over 3}} \right)^n}\,\, \Rightarrow \,\,0.1666 > {\left( {{2 \over 3}} \right)^n}$

${n_{\min }} = 5$
3

JEE Main 2019 (Online) 11th January Morning Slot

Two integers are selected at random from the set {1, 2, ...., 11}. Given that the sum of selected numbers is even, the conditional probability that both the numbers are even is
A
${2 \over 5}$
B
${1 \over 2}$
C
${7 \over 10}$
D
${3 \over 5}$

Explanation

Since sum of two numbers is even so either both are odd or both are even. Hence number of elements in reduced samples space = 5C2 + 6C2

so required probability = ${{{}^5{C_2}} \over {{}^5{C_2} + {}^6{C_2}}}$
4

JEE Main 2019 (Online) 11th January Evening Slot

Let  S = {1, 2, . . . . . ., 20}. A subset B of S is said to be "nice", if the sum of the elements of B is 203. Then the probability that a randonly chosen subset of S is "nice" is :
A
${5 \over {{2^{20}}}}$
B
${7 \over {{2^{20}}}}$
C
${4 \over {{2^{20}}}}$
D
${6 \over {{2^{20}}}}$

7,

1, 6

2, 5

3, 4

1, 2, 4