LCM - just list out all the next multiples of the number and the first one to match both is the LCM, which is the simplest way to do and understand.
3. eight goes into 56, so the LCM is 56
4. again, 25 goes into 50 so 50 is the LCM
5. 12 24 36 48 60 | 30 60, so 60 is the LCM
6. 6 12 18 24 30 | 10 20 30, so 30 is the LCM
7. 16 32 48 | 24 48, so 48
8. 14 28 56 70 84 | 21 42 63 84, so 84
9. 9 18 21 36 45 | 15 30 45, so 45
10. 5... 55 | 11.. 55 so 55
11.
a) LCM of 3 and 5, which is the 15th day
b) The next common multiple of 3 and 5 is 30, so the 30th day. Since there are less than 30 days in February, this means that Kevin will not water both plants together again in February.
Hope I helped!