Stage: 3 Challenge Level: 
D = 2pqr where p, q and r are primes and there are only a few cases to consider as p, q and r can't be very big.
Creating expressions/formulae. Mathematical reasoning & proof. Prime factors. Using symbols. Time. Generalising. Visualising. Interactivities. Odd and even numbers. smartphone.
Published March 2002,April 2002.
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