To find the probability of an event occurring, we must take the favorable outcomes (what we want to happen) over all of the possible outcomes (all of the things that could happen).
Therefore, there are four alarm clocks, so the amount of favorable outcomes is 4.
The total number of clocks is 10, so the number of possible outcomes is 10.
The probability that a randomly selected clock will be an alarm clock is 4/10, which when simplified equals 2/5 or 40%.
Hope this helps!