I made a mistake in my second example
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Second example if your swimmer trains 50m intervals and can hold 40s then 2 turns are already included in the interval and
you have to add 1 turn only to your 100m race prediction: 2*40s + 1.2s – 1.7s = 81.9s
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The correct answer is 2*40s + 1.2s – 1.7s = 79.5s