
Kenneth S. answered 03/30/18
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Expert Help in Algebra/Trig/(Pre)calculus to Guarantee Success in 2018
you ought to know the various (small) powers of 2, 3, 5 and a few others--most people would then be able to recognize instantly that 27 = three cubed.
failing that, you can examine 27 and see that it's divisible by three since the sum of its digits is divisible by 3.
repeated division by smaller primes would enable you to prime-factorize ANY composite number.