1. The problem statement, all variables and given/known data In an arithmetic progression, the sum of the first 10 terms is the same as the sum of the next 5 terms. Given that the first term is 12, find the sum of the first 15 terms. 2. The only one I could think of is S= n/2 (2a+(n-1)d) 3. I've tried solving it, but failed. I guess this is due to lack of experience, as I missed the topic.