156221

First of all I put the first number that I was aiming for, which was 15:
15
Then I found two numbers that made 15:
15
9 6
Then I made two numbers that made nine and found one number that went with three to make six:
15
9 6
6 3 3
Then I made two numbers that made six, another number that would go with the second number and make 3 and a fourth number that made 3 with the third number:
15
9 6
6 3 3
4 2 1 2
Here are ten other sets I made of the bottom four numbers: 1. 3213 2. 2124 3.3123 4. 6113 5. 5114 6. 1132 7. 4115 8. 2311 9. 2311 10. 2131