5.D.4. TIMING WITH HOURGLASSES
I have just started these and they are undoubtedly older than the examples here. I don't recall ever seeing a general approach to these problems.
Simon Dresner. Science World Book of Brain Teasers. 1962. Op. cit. in 5.B.1. Prob. 17: Two minute eggs, pp. 9 & 87. Time 2 minutes with 3 & 5 minute timers.
Howard P. Dinesman. Superior Mathematical Puzzles. Op. cit. in 5.B.1. 1968. No. 21: The sands of time, pp. 35 & 93. Time 9 minutes with 4 & 7 minute timers.
David B. Lewis. Eureka! Perigee (Putnam), NY, 1983. Pp. 73 74. Time 9 minutes with 4 & 7 minute timers.
Yuri B. Chernyak & Robert S. Rose. The Chicken from Minsk. BasicBooks, NY, 1995. Chap. 1, prob. 8: Grandfather's breakfast, pp. 6 & 102. Time 15 minutes with 7 & 11 minute timers.
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