find a common rule that applies to every input->output
1. divide by 3
15/3=5
24/3=8
2. multiply by 9
2*9=18
6*9=54
3. add 12
10+12=22
15+12=27
4. subtract 7
32-7=25
36-7=29
5. divide by 7
14/7=2
35/7=5
6. add 13
13+13=26
15+13=28
