Maths Olympiad Prep

Track / Stage 6 / 292 of 400 #1292 of 1964

Problem 1292

National olympiad, first round
Combinatorics Difficulty 6.5 Find the answer

A fair coin is to be tossed 1010 times. Let i/ji/j, in lowest terms, be the probability that heads never occur on consecutive tosses. Find i+ji+j.

A number or a short expression. Spacing, $ signs and \frac vs / are all fine.

Official solution

1. Define the problem in terms of sequences:
We need to find the number of sequences of length 10 consisting of heads (H) and tails (T) such that no two heads are consecutive.

2. Use dynamic programming to count valid sequences:
Let an a_n be the number of valid sequences of length n n ending in T, and bn b_n be the number of valid sequences of length n n ending in H. The total number of valid sequences of length n n is an+bn a_n + b_n .

3. Establish recurrence relations:
- If a sequence of length n n ends in T, the previous n1 n-1 sequence can end in either H or T. Thus, an=an1+bn1 a_n = a_{n-1} + b_{n-1} .
- If a sequence of length n n ends in H, the previous n1 n-1 sequence must end in T (to avoid consecutive heads). Thus, bn=an1 b_n = a_{n-1} .

4. Initial conditions:
- For n=1 n = 1 , we have a1=1 a_1 = 1 (sequence: T) and b1=1 b_1 = 1 (sequence: H).

5. **Compute the values up to n=10 n = 10 :**
a2=a1+b1=1+1=2,b2=a1=1,a3=a2+b2=2+1=3,b3=a2=2,a4=a3+b3=3+2=5,b4=a3=3,a5=a4+b4=5+3=8,b5=a4=5,a6=a5+b5=8+5=13,b6=a5=8,a7=a6+b6=13+8=21,b7=a6=13,a8=a7+b7=21+13=34,b8=a7=21,a9=a8+b8=34+21=55,b9=a8=34,a10=a9+b9=55+34=89,b10=a9=55. \begin{align*} a_2 &= a_1 + b_1 = 1 + 1 = 2, \\ b_2 &= a_1 = 1, \\ a_3 &= a_2 + b_2 = 2 + 1 = 3, \\ b_3 &= a_2 = 2, \\ a_4 &= a_3 + b_3 = 3 + 2 = 5, \\ b_4 &= a_3 = 3, \\ a_5 &= a_4 + b_4 = 5 + 3 = 8, \\ b_5 &= a_4 = 5, \\ a_6 &= a_5 + b_5 = 8 + 5 = 13, \\ b_6 &= a_5 = 8, \\ a_7 &= a_6 + b_6 = 13 + 8 = 21, \\ b_7 &= a_6 = 13, \\ a_8 &= a_7 + b_7 = 21 + 13 = 34, \\ b_8 &= a_7 = 21, \\ a_9 &= a_8 + b_8 = 34 + 21 = 55, \\ b_9 &= a_8 = 34, \\ a_{10} &= a_9 + b_9 = 55 + 34 = 89, \\ b_{10} &= a_9 = 55. \end{align*}

6. Total number of valid sequences of length 10:
a10+b10=89+55=144. a_{10} + b_{10} = 89 + 55 = 144.

7. Total number of possible sequences:
210=1024. 2^{10} = 1024.

8. Probability that heads never occur on consecutive tosses:
1441024=964. \frac{144}{1024} = \frac{9}{64}.

9. Sum of the numerator and denominator:
i+j=9+64=73. i + j = 9 + 64 = 73.

The final answer is 73 \boxed{73} .

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.