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IAI Actuarial Core Principles · Risk Modelling and Survival Analysis · Stochastic processes

A symmetric simple random walk starts at 2 with absorbing barriers at 0 and 5. What is the probability it is absorbed at 5?

The probability is 0.4. For a symmetric walk, the chance of reaching the upper barrier first equals the starting position divided by the barrier distance, 2 over 5, since the walk is a martingale.

  1. A0.2
  2. B0.3
  3. C0.4Correct
  4. D0.5
  5. 0.6

Explanation

For a symmetric walk the gambler's ruin probability of hitting the upper barrier is start/upper = 2/5 = 0.4. The value 0.6 is the probability of ruin at 0, which is the complement.

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