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First, we must calculate the cash flow yield. This statistic is calculated just like the IRR:
504.13 = 100 / (1 + r) + 500 / (1 + r)^2
r = 10%
The Macaulay duration is the weighted average of time to receipt of each payment, discounted at the cash flow yield:
MacDur = [1(100) / 1.1 + 2(500) / 1.1^2] / (100 / 1.1 + 500 / 1.1^2) = 1.82