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Factors and Multiples

Disclaimer: This article is open for discussion. I have bought this reference book for Secondary 1 Express Mathematics, fourth reprint 2012, 2nd Edition, and found some questionable examples.

Highest common factor

Highest common factor

In the above extract from Page 2, 3 is not a factor of 20. The solution should be as below:

Factors of 18 are 1, 2, 3, 6, 9, 18
Factors of 20 are 1, 2, 4, 5, 10, 20
Common factors of 18 and 20 are 1 and 2.
Hence HCF of 18 and 20 is 2.

HCF = highest common factor.

I am not sure if they have rectified the error, just want to alert parents and students if you have bought the same book with the same version as mine. I welcome comments and feedback. Thank you.