Question

A frog moves in a sequence of unit steps. Each step is N, S, E or W with equal probability. It starts at the origin. Find the probability that it reaches (2, 2) in less than 7 steps.

Answer 1

The answer is 067.

It takes an even number of steps for the object to reach (2,2), so the number of steps the object may have taken is either 4 or 6 .

If the object took 4 steps, then it must have gone two steps $\mathrm{N}$ and two steps E, in some permutation. There are $\frac{4 !}{2 ! 2 !}=6$ ways for these four steps of occuring, and the probability is $\frac{6}{4^{4}}$.

If the object took 6 steps, then it must have gone two steps N and two steps E, and an additional pair of moves that would cancel out, either N / S or W/E. The sequences N, N, N, E, E, S can be permuted in $\frac{6 !}{3 ! 2 ! 1 !}=60$ ways. However, if the first four steps of the sequence are N, N, E, E in some permutation, it would have already reached the point (2,2) in four moves. There are $\frac{4 !}{2 ! 2 !}$ ways to order those four steps and $2 !$ ways to determine the order of the remaining two steps, for a total of 12 sequences that we have to exclude. This gives $60-12=48$ sequences of steps. There are the same number of sequences for the steps N, N, E, E, E, W, so the probability here is $\frac{2 \times 48}{4^{6}}$.

The total probability is $\frac{6}{4^{4}}+\frac{96}{4^{6}}=\frac{3}{64}$, and $m+n=067$.

What is probability?

• Probability is synonymous with possibility. It is a mathematical discipline that deals with the occurrence of a random event. The value ranges from zero to one.
• Probability has been introduced in mathematics to predict the likelihood of occurrences occurring. Probability is defined as the degree to which something is likely to occur.
• This is the fundamental probability theory, which is also utilized in probability distribution, in which you will learn about the possible results of a random experiment.
• To determine the likelihood of a particular event occurring, we must first determine the total number of alternative possibilities.

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