Answer:
3hrs 12 mins
Step-by-step explanation:
For every 10% charge she gets 1hr 20mins (or 80 mins)
Charge drop = (81% - 57%) = 24%
Since 10% gives 80 mins;
     24% gives (24/10) x 80 mins = 192 mins
since 1hr = 60 mins
192 mins = 192/60 = 3hrs 12 mins