How do you draw a flow chart to check whether a given number is prime or not?

There are flowcharts available for computers, that depend upon brute number crunching. In other Words, they are dividing a given number by every lesser number and seeing if there are any that divide evenly into it.

For humans, this is a bit harder. However, here are four steps to let you know if a number can be divided by the numbers 1 through 9 (but not seven).

1. Is it an even number? If yes, then stop, it's not prime.

2. Is it a number ending in zero or five? If yes, then stop, it's not a prime.

3. Do the digits in the number add up to three or nine? If yes, then stop, it's not a prime.

Now, any "yes" answer ends it. But if all the above were answered "no", that doesn't actually end it. Maybe the number can be divided by 7. Or 11. Or even larger numbers.