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IAI Actuarial Core Principles · Economic Modelling · Black-Scholes derivative-pricing model

A share follows GBM with S_0 = 100, mu = 10% p.a. and sigma = 30% p.a. What is the median of S_1 (the value exceeded with probability 50%), to the nearest rupee?

The median equals S_0 times e to the power (mu minus sigma squared over 2) times t, which is 100 times e to the power 0.055, about Rs 106. It lies below the mean of Rs 110.5 because of lognormal skewness.

  1. ARs 111
  2. BRs 106Correct
  3. CRs 110
  4. DRs 115
  5. Rs 100

Explanation

ln S_1 is normal with mean ln100 + (0.10 - 0.045) = ln100 + 0.055. The median is 100 e^{0.055} = 105.65, about Rs 106. Rs 110 uses the mean e^{0.10} ignoring the Ito term, and Rs 111 is the same figure rounded up from 110.52.

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