Answer:
6+15+21=42.
Step-by-step explanation:
21 is greater than 16
15 is less than 29
1*21=21
3*7=21
7*3=21
factors of 21 are 1, 3, 7 and 21
1*15=15
3*5=15
5*3=15
factors of 15 are 1, 3, 5 and 15
6+15+21=42
1*6=6
2*3=6
3*2=6
factors of 6 are 1, 2, 3 and 6