Timer 1

K > 20000000 / (65536 * 5)
K > 61.035
K = 64

PR1 = ((20000000 / 64) / 5) - 1 = 62499

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Timer 3

K > 20000000 / (65536 * 25)
K > 12.207
K > 16

PR3 = ((20000000 / 16) / 25) - 1 = 49999