Question

When Famke charges her phone, it gains 2 percentage points each minute. When Frmke uses her phone, it loses 5 percentage points each minute. Once, Famke used her phone for some time, and then charged it for twice the amount of time she used it. In the end, her phone lost 10 percentage points. How long did Famke use her phone and how long did she charge it?

Answer 1

Charging: +2/min percentage points.
Using: -5/min percentage points.
Amount of time Famke used her phone: x
Amount of time Famke charge her phone: 2x
Famke gained 2x*(2/min)=x*(4/min) percentage points.
Over this period her phone lost 10 percentage points→
x*(5/min)-x*(4/min)=10⇒x*(1/min)=10
x=10

This means that she used her phone for 10 minutes and charged it for 20 minutes.