Answer:
62.5%
Explanation:
Split the time up into 5 minute slots. That gives us 24 slots of 5 minutes.
20/5=4 of the 5 minute slots playing outside
25/5=5 of the 5 minute slots on his phone
5+4=9 out of 24 slots spent
24-9=15 slots remaining to play his instrument
15/24x100=62.5% time playing instrument