Logs are stacked 7 rows high. the top row has 15 logs, the second row has 18 logs, and the third row has 21 logs. this pattern continues similarly.

what is the total number of logs in the stack?

The answer is 168 logs.

1st row:          15 logs
2nd row:        18 logs
3rd row:         21 logs
(so, each row has 3 more logs)
4th row:         24 logs
5th row:         27 logs
6th row:         30 logs
7th row:         33 logs

The sum of logs from each row is: 15 + 18 + 21 + 24 + 27 + 30 + 33 = 168 logs

The answers are 20,30,80,180,960,890,1230,4560. on the first row