Answer :
GCF (Greatest Common Factor)
6: 1,2,3,6
20: 1,2,4,5,10,20
2 is the greatest number in both, therefore there can only be 2 groups of 3 sopranos, and 10 altos.
6: 1,2,3,6
20: 1,2,4,5,10,20
2 is the greatest number in both, therefore there can only be 2 groups of 3 sopranos, and 10 altos.