A telephone tree is used to send messages. The person at the top calls 2 people. Each person calls 2 more people. Suppose it takes 1 min to call someone. A message is relayed until the bottom row of the tree has 256 people. How long did this take? How do you know?
It takes 16 minutes to call 256 people. So, one person calls 2 people which took him/her 2 minutes, 1 minute for each call. So, now those two people call two more people each, which took them 2 more minutes. All this goes on till the have called 256 people.
So, if everyone calls two more people when they were called. Altogether it would take them 16 minutes.