LCM - Lowest common multiple

This is the smallest number that will divide by the given numbers.

For example, the LCM of 5 and 4 is? 

x5:          5          10          15          20          25…

x4:          4          8             12          16          20…

20 is the lowest number that is divisible by both 5 and 4, making it the LCM.


HCF - Highest common factor

This is the biggest number that will divide into the given numbers

For example, the HCF of 36 and 54 is?

factors of 36:  1      2      3      4      6      9      12      18      36
factors of 54:  1      2      3      6      9     18    27      54

18 is the highest number that will divide into 36 and 54, making it the HCF.