Prime numbers are numbers that are only divisible by 1 and themselves. So, to tell if a number is prime, look at its factors.
19:
1 x 19
29:
1 x 29
39:
1 x 39
3 x 13
So, because 39 has more than one set of factors, it's NOT a prime number.
The fact that others do not help in this case is best explained by "bystander apathy".<span>
The bystander effect, or bystander apathy, is a social mental wonder in which people are less inclined to offer assistance to a casualty when other individuals are available. The more noteworthy the quantity of onlookers, the more improbable it is that any of them will help.
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