Answer: 5 + 5^2 + 5^3 + 5^4
Explanation:
1) First time, when Kris forwards the e-mail it reaches other 5 people.
2) Second time, each one of the 5 people forawer to other 5 people, meaning that the second time the e-mail has reached other 5^2 people.
3) Third time, each 5^2 people forwarded to other 5 people, so the email reaches other 5^3 people
4) Fourth time => 5^4 new people.
So, the total number (not including Kris) is 5 + 5^2 + 5^3 + 5^4.