Enova

www.enova.com
Engaged Employer

## Interview Question

Interview Chicago, IL

# A few years ago, Sprint decided that they will make phone

calls from cellphone-cellphone free! There is a legend that a guy who was 20 years old by that time decided to call every single cellphone in the united states, and when he was 80 years old he finished doing all that calls. Is that possible? prove it!

2

Start with Assumptions: 320 million US citizens, avg life 80 years, he is calling people living at this time (in 20 years he isnt calling the new 20 year olds), the us population is equally distribed, same number of people are in the age range 0-10 as 70-80. Break it down to age groups to assume total size of cell phone market in the US Each age group has 40 million people. 0-10 0% have cell phones 10 - 20 25% 20 - 30 50% 30 - 40 50% 40 - 50 50% 50 - 60 25% 60 - 70 25% 70 - 80 10% that gives you a total of 104 million cell phones. now assume that he uses 30 seconds per call, he can call 2 people per minute, he needs 104 million / 2 = 52 million minutes to do this. Now assume that 60 years, he makes calls 360 days a year, 10 hours a day. thats 60 years* 360 days / year * 10 hours / day* 60 minutes / hour = 12,960,000 minutes. No this is not feasible. It all depends on the numbers you use, these are conservative but they want to know how you think and work through the problem.

Anonymous on Jul 19, 2012
0

Nice approach but there is no need to split the population into age groups. You could just roughly estimate the number of cell fones. The number of calls that a person could make in a worst case scenario would be around 16M. This way below the #people having cell fones by any reasonable estimate.

csd on Dec 16, 2012