Consider the question. The file, 90 megabytes in size, downloads at the rate of 5 megabytes per second for its first 60 megabytes, and then 10 megabytes per second thereafter. How long, in seconds, does it take to download entirely?
 If the answer is '15'; explain the reasoning:
The first 60 megabytes take 60 / 5 = 12 seconds. There are 90 - 60 = 30 remaining megabytes. The remaining 30 megabytes take 30 / 10 = 3 seconds. And 12 + 3 = 15 seconds.