Today Puzzle #579
Puzzle No. 579鈥� Tuesday 1 October
Last week Cambridge University waived a fine for a library book returned 60 years late. They impose a fine of £1.50 per week and suggested the fine would have been £4700. However, if they added weekly interest of 0.1%, how much would it have been?
Today’s #PuzzleForToday has been set by Dr Shaun Fitzgerald, Director of The Royal Institution
Click here for the answer
The answer is the sum s of a geometric series given by s= a x (1-rn) / (1-r), where a is £1.50, r is the ratio of the weekly difference (1+0.1%)=1.001, and n is the number of weeks = 60 x 52 = 3120.
s = 1.5 x (1 – 1.0013120) / (1 – 1.001) = £32,417.
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