Provide each student with a hundreds board and 2 colors of counters. (At least 5 counters of each color.
Begin by showing students two fractions that are to be added (ex. 1/3 and 1/4). Remind students that fractions of different sized pieces may not be combined. The idea is to find a multiple that the two denominators have in common.
Using one color of counters, cover the first 5 multiples of the first denominator on the hundreds board (3,6,9,12,15).
Leave these on the board. Then use the other color and cover the first 5 multiples of the other denominator on the hundreds board (4,8,12,16,20).
The first number that is covered by 2 counters is the common multiple that you have been looking for!
As we try to look for common multiples, we keep a list on the board. After a while the students realize that when one denominator is a multiple of the other, then the LCM is the larger number!