Here's my guess:
Start at 12, add 3 to make it 15 (divisible by 5).
Add another five to get it to next number divisible by 5 (20).
Add 10 to get it to 30 (last number divisible by five).
Add 2 to get to desired number (32).
Take all addition numbers (+3, +5, +10, +2) and add them...