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JEE Advanced 2020 · Probability (Class 12)

Two fair dice, each with faces numbered 1,2,3,4,51, 2, 3, 4, 5 and 66, are rolled together and the sum of the numbers on the faces is observed. This process is repeated till the sum is either a prime number or a perfect square. Suppose the sum turns out to be a perfect square before it turns out to be a prime number. If pp is the probability that this perfect square is an odd number, then the value of 14p14p is ______\_\_\_\_\_\_.

Answer: 8.008.00

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Written solution

Two fair dice; observe the sum\text{Two fair dice; observe the sum}

Repeat till the sum is prime or a perfect square\text{Repeat till the sum is prime or a perfect square}

Given: the stop is a perfect square before a prime\text{Given: the stop is a perfect square before a prime}

p=P(odd square∣square before prime)p = P(\text{odd square} \mid \text{square before prime})

To find: 14p\text{To find: } 14p

Sums: 2,3,4,5,6,7,8,9,10,11,12\text{Sums: } 2,3,4,5,6,7,8,9,10,11,12

Primes: 2,3,5,7,11\text{Primes: } 2, 3, 5, 7, 11

Perfect squares: 4, 9\text{Perfect squares: } 4,\ 9

Prime sums: 2:1, 3:2, 5:4, 7:6, 11:2\text{Prime sums: } 2{:}1,\ 3{:}2,\ 5{:}4,\ 7{:}6,\ 11{:}2

Total=1+2+4+6+2=15\text{Total} = 1+2+4+6+2 = 15

P(prime)=1536P(\text{prime}) = \dfrac{15}{36}

Square sums: 4:3, 9:4⇒3+4=7\text{Square sums: } 4{:}3,\ 9{:}4 \Rightarrow 3+4 = 7

P(square)=736P(\text{square}) = \dfrac{7}{36}

Others (repeat): 6:5, 8:5, 10:3, 12:1\text{Others (repeat): } 6{:}5,\ 8{:}5,\ 10{:}3,\ 12{:}1

P(repeat)=1436P(\text{repeat}) = \dfrac{14}{36}

Re-rolls are irrelevant⇒race is prime vs square\text{Re-rolls are irrelevant} \Rightarrow \text{race is } \text{prime vs square}

P(square before prime)=7/367/36+15/36=722P(\text{square before prime}) = \dfrac{7/36}{7/36 + 15/36} = \dfrac{7}{22}

Given a square: 4→3 ways,9→4 ways\text{Given a square: } 4 \to 3 \text{ ways},\quad 9 \to 4 \text{ ways}

Odd square=9 only\text{Odd square} = 9 \text{ only}

p=43+4=47p = \dfrac{4}{3+4} = \dfrac{4}{7}

14p=14×47=814p = 14 \times \dfrac{4}{7} = 8

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