3 Answers

1 and 5

To find least common multiple of 10 and 20, let us first list down the multiples of 10 and 20.
Multiples of 10 are 10, 20, 30, 40, 50, 60, ..... Multiples of 20 are 20, 40, 60, 80, 100, .....
The multiples common between the two are 20, 40, 60, ....
The least of these common multiples is 20 and hence the answer.
Next one is to find LCM of 3, 5 and 6
Multiples of 3 are 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 45, 48, 51, 54, 57, 60, ....
Multiples of 5 are 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, ...
Multiples of 6 are 6, 12, 18, 24, 30, 36, 42, 48, 54, 60, ....
The common multiples of the three numbers 3, 5 and 6 are 30, 60, ...
The least of these common multiples is 30.

100, 200...

I'll say five, but there more of them!