It takes Cassie fourteen minutes to load a certain video o

Assuming total amount of work always helps in such problems. Total work should be multiple of each of the given time.

Time taken to load video on cellphone = 14 minutes = 14*60 seconds
Times taken to load video on computer= 14 seconds
Let total amount of work= 14*60 units
Rate of loading video on cell phone = Total work/ time taken= 14*60/14*60 = 1 unit/sec
Similarly rate for computer= 14*60/14 = 60 Units/ sec
Combined rate = 1+60 = 61 seconds
Combined time = Total work/ combined rate = (14*60)/ 61 = 13.77

You don't even have to do the division. If denominator was 60 then answer would be 14. Since it is slightly greater than 60 then look for an option which is slightly less than 14.

Yes. I also chose the ans with approximation guess. As it is clear the majority of the download will be done by laptop but phone will also do very small portion of work. Therefore, ans should be less than 14 sec but should be very close to 14 sec i.e. 13.77

Rate(Cellphone) = 1/(14*60)
Rate ( Laptop) = 1/14

Combined Rate= 61/(14*60)

Rate * Time = Work

61/(14*60) * Time = 1

Time = 13.77

phone - 840 secs
laptop - 14 sec

so laptop 60 times more productive
if use both
840 = 60x + x
x= 840/61x = 13.77

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