While playing games on a particular smart phone, 1% of the battery is drained every five minutes of game play. The battery starts off at 80% full.

Question

The number of minutes played on a full battery

 
The amount of battery needed to play for 80 minutes

 
The amount of battery needed to play for 75 minutes

 
None of these

Answer 1

If 1% of battery capacity is drained for each 5 minutes of play,
   then a full battery (100%) can last for (1%*5 minutes/percent) = 500 minutes     (note that percent cancels out)
 
80 minutes / 500 minutes   =  0.16 = 16%
 
75 minutes / 500 minutes  =  0.15  = 15%
 
Sorry, what is "none of these?"
 
Extra note: this is only a problem, so don't expect a linear draining of a real battery or the ability to drain to 0.