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'Ten Green Bottles' printed from http://nrich.maths.org/
Why do this problem?
is one which could be done quickly as an introduction when extending or revising work on time and clocks.
When does the first bottle fall?
So when does the second bottle fall?
How many $5$ minutes are there between the first and tenth bottles falling?
Learners could find the equation for the $nth$ bottle falling.
Suggest using a real or model clock and counting.