HCF:
Use prime numbers
First try to divide the number by 2
If it does not give u a whole number, try the next prime number, 3
HCF of 54 and 72
54=2x3x3x3=2¹x3³
72=2x2x2x3x3=2³x3²
Look at the power for the prime number 2
54’s power is lower at number 1 instead of 3 as for 72
Take the lower power for each prime and multiply
So HCF=2¹x3²=18

LCM also can use prime numbers
54 =2¹x3³
72=2³x3²
Except this time pick the larger power of the two options
For number 2, 72 has the larger number, hence use 2³, while for number 3, 54 has the larger power, hence use 3³
LCM=2³X3³=216

FYI: Power refers to the small floaty number, which represents the number of that particular number multiplied together. Eg, 8=2x2x2 has three 2s so 8=2³, with the power of 3

Prime numbers are numbers divisible by 1 and itself only, including 1,2,3,5,7,11,13,17,19...