assuming that you want to add or subtract them and want the bottom number the same,
1/4 and 2/3
least common multiple of 4 and 3
4=2 times 2
3=3 times 1
so the leas common mulltiple will include all of them
the number is 2 times 2 times 3 or 12
lcm=12
1/4 times 3/3=3/12
2/3 times 4/4=8/12
add or subtract 3 and 8
LCM=12
