Let's make a table of the days, and the number of mails sent each day:
Day nr of mails sent
Monday 9
Tuesday ?
Wednesday 13
Thursday 15
Friday 17
In the last 3 days, the number of mails increased each day by 2, so we could make the assumption that on Tuesday, there were 2 mails less sent than on Wednesday.
So, on Tuesday, we assume that 13-2=11 mails were sent.
We check this assumption with the number of mails sent on Monday, and it works again. So, our assumption is correct.
Answer: 11