Kris forwards an e-mail to 5 people. each of those people forward that e-mail to 5 more people. if this happens 2 more times, which expression shows the total number of people who have recived the e-mail, not including kris

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.