Q46 / 75 · JEE Main 2026 (04 April Shift 1) · Mathematics
A coin is tossed $8$ times. If the probability that exactly $4$ heads appear in the first six tosses and exactly $3$ heads appear in the last five tosses is $p$, then $96p$ is equal to _____.
Correct answer: 9
Solution
Let the outcomes of the $8$ tosses be denoted by $X_1, X_2, X_3, X_4, X_5, X_6, X_7, X_8$.
Let $A$ be the number of heads in the first $3$ tosses ($X_1, X_2, X_3$).
Let $B$ be the number of heads in the next $3$ tosses ($X_4, X_5, X_6$).
Let $C$ be the number of heads in the last $2$ tosses ($X_7, X_8$).
According to the given conditions:
Number of heads in the first six tosses: $A + B = 4$
Number of heads in the last five tosses: $B + C = 3$
Since $C$ is the number of heads in $2$ tosses, $C \le 2$. Thus, $B = 3 - C \ge 1$. Also, $B \le 3$.
The possible values for $B$ are $1, 2, 3$. We analyze each case:
Case 1: $B = 1$
Then $C = 2$ and $A = 3$.
Number of ways = $^{3}C_{3} \times ^{3}C_{1} \times ^{2}C_{2} = 1 \times 3 \times 1 = 3$
Case 2: $B = 2$
Then $C = 1$ and $A = 2$.
Number of ways = $^{3}C_{2} \times ^{3}C_{2} \times ^{2}C_{1} = 3 \times 3 \times 2 = 18$
Case 3: $B = 3$
Then $C = 0$ and $A = 1$.
Number of ways = $^{3}C_{1} \times ^{3}C_{3} \times ^{2}C_{0} = 3 \times 1 \times 1 = 3$
Total number of favorable outcomes = $3 + 18 + 3 = 24$
Total number of possible outcomes for $8$ coin tosses = $2^8 = 256$
The probability $p$ is given by:
$p = \dfrac{24}{256} = \dfrac{3}{32}$
We need to find the value of $96p$:
$96p = 96 \times \dfrac{3}{32} = 3 \times 3 = 9$
Answer: $9$
Let $A$ be the number of heads in the first $3$ tosses ($X_1, X_2, X_3$).
Let $B$ be the number of heads in the next $3$ tosses ($X_4, X_5, X_6$).
Let $C$ be the number of heads in the last $2$ tosses ($X_7, X_8$).
According to the given conditions:
Number of heads in the first six tosses: $A + B = 4$
Number of heads in the last five tosses: $B + C = 3$
Since $C$ is the number of heads in $2$ tosses, $C \le 2$. Thus, $B = 3 - C \ge 1$. Also, $B \le 3$.
The possible values for $B$ are $1, 2, 3$. We analyze each case:
Case 1: $B = 1$
Then $C = 2$ and $A = 3$.
Number of ways = $^{3}C_{3} \times ^{3}C_{1} \times ^{2}C_{2} = 1 \times 3 \times 1 = 3$
Case 2: $B = 2$
Then $C = 1$ and $A = 2$.
Number of ways = $^{3}C_{2} \times ^{3}C_{2} \times ^{2}C_{1} = 3 \times 3 \times 2 = 18$
Case 3: $B = 3$
Then $C = 0$ and $A = 1$.
Number of ways = $^{3}C_{1} \times ^{3}C_{3} \times ^{2}C_{0} = 3 \times 1 \times 1 = 3$
Total number of favorable outcomes = $3 + 18 + 3 = 24$
Total number of possible outcomes for $8$ coin tosses = $2^8 = 256$
The probability $p$ is given by:
$p = \dfrac{24}{256} = \dfrac{3}{32}$
We need to find the value of $96p$:
$96p = 96 \times \dfrac{3}{32} = 3 \times 3 = 9$
Answer: $9$
Hence, the correct answer is 9.