Why is it very common to have a 9 minute snooze interval on
alarm clocks, why not 10 instead?
By setting the snooze time to 9 minutes, the alarm clock only needs to
watch the last digit of the time. So, if you hit snooze at 6.45, the alarm goes
off again when the last digit equals 4. They couldn't make it 10 minutes,
otherwise the alarm would go off right away, or it would take more circuitry.