Well, first of all, LCM means Lowest Common Multiple. So you have to write out the factors of each of the numbers. So that will be;
2= 2, 4, 6, 8, 10, 12, 14, 16, 18, 20
5=5, 10, 15, 20
So now, you have to pick out the numbers that are common to both numbers. So;
10, 20
10 and 20 are the only common numbers. So you have to look for the lowest common number. And that is 10. So the LCM is 10.. Hope i helped. Have a nice day.