let's say t is the amount of time she used it, then it lost t*(5/min) percentage points. then, she charged it for 2t, so it gained (2t)*(2/min) =t*(4/min) percentage points.
this means that above this time period, the phone lost t*(5/min)-t*(4/min) =t*(1/min) percentage points; because this loss is recognized to be 10 percentage points, this means t*(1/min) =10, so t=10 min.
famke used her phone for 10 minutes and charged it for 20 minutes.
- cherry :)
