<human>:  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?
<bot>: How many seconds do the first 60 megabytes take? ** The first 60 megabytes take 60/5=<<60/5=12>>12 seconds.
How many megabytes are remaining? ** There are 90-60=<<90-60=30>>30 remaining megabytes.
How many seconds do the remaining megabytes take? ** The remaining 30 megabytes take 30/10=<<30/10=3>>3 seconds.
How long, in seconds, does the file take to download entirely? ** And 12+3=<<12+3=15>>15 seconds.
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