I factor each and then use each prime factor the most it was used in either number.
40=2x2x2x5
12=2x2x3
LCM=2x2x2x3x5=120
You can also write out the multiples of each until you have one that is in both lists.
12,24,36,48,60,72,84,96,108,120
40,80,120
the LCM of 40 and 12 is 120.
LCM of 12, 24, and 40 = 120
The LCM is 2520.
The LCM is: 840
120
120
120
2280
It is: 600
120
It is: 120
It is 120