What is the least common denominator for 5/6 and 3/8. 3/4 and 6/7

You need to find the least common multiple of the denominators. Check:
6+6=12     6+6+6=18      6+6+6+6=24    
8+8=16     8+8+8=24      8+8+8+8=32    
Now we see that the least common multiple is: 24.

Similarly, we proceed to the second:
4+4=8       4+4+4=12     4+4+4+4=16     4+4+4+4+4=20      4+4+4+4+4+4=24    4+4+4+4+4+4+4=28    
7+7=14     7+7+7=21     7+7+7+7=28
again, we see that the least common multiple is: 28.

Igot 108 hope that’s right !