I have 4 hourglasses: Hourglass A measured 7 minutes, Hourglass B measured 11 minutes, Hourglass C measured 23 minutes, Hourglass D measured 37 minutes.
If the objective is to minimize the number of times to turn the hourglasses, which 2 of the 4 hourglasses should I choose if I want to measure 71 minutes?
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23×4 - 7×3 = 71
turn 4 + 3 = 7 times