The GCF is the smallest shared prime factor for each term.
The LCM is the smallest whole number that both numbers share as multiples.
For example, take #16.
Factors of 25: 1 , 5, 25
Factors of 75: 1 , 3, 5, 15, 25 ,75
The GCF that is shared would then be 25.
The smallest multiple that both share as multiples:
Multiples of 25: 25, 50, 75, etc.
Multiples of 75: 75, etc.
The LCM that is shared would then be 75.
16) 25 and 75
GCF: 25
LCM: 75
17) 35 and 5
GCF: 5
LCM: 35
18) 27 and 36
GCF: 9
LCM: 108
19) 21 and 56
GCF: 7
LCM: 168
20) 54 and 72
GCF: 18
LCM: 216