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I am now wondering where i should ask my questions at, which forum. Its pre-algabra so i dont want to bother anyone with some really basic things that they dont care to see/

For example, the question i have right now is a method for simply finding a composite number. Im going through the khan Academy exercises and they have this......

Which of these numbers is composite?

71, 79, 59, 91, 83

Now i know what composite is ( a number that has a factor of itself, 1 and other numbers ), i know what prime is ( a number with a factor of 1 and itself ). But the thing i cant ask anyone is that i dont understand their method at arriving at the answer.

The only way i can think to figure the answer is to do this for every number...

71/1 = 71

71/2 = 35r1

71/3 = 23r2

71/4 = 17r3

71/5 = 14r1

71/6 = 11r1

71/7 = 10r1

How long do i keep this up before i can say, yes this is a prime number and move on to the next number. The answer is given and its 91 is composite cause 1x71= 71 and 7x13= 91. But something tells me im going about it all wrong.

thank you for any advice.