Well contrary to popular belief that the we only use about 5-10% of our brain, we actually use 100% percent of our brain.
There isn't a single part of our brain than isn't in use, we might not use every part of the brain at once but we most certainly use all of it. As far as how much of our brain power we use, that's a completely different thing entirely.
The brain is basically like a CPU, to carry-out day to day activities like walking and talking, the brain uses minimal brain power when it comes to more difficult tasks like solving match problems or solving a rubik's cube, we use a significantly higher amount of brain power.
The amount of brain power people use varies from person to person as well as from task to task so even though we might not use all of our brain at once, we certainly use all of it at some point in time.
As far as brain power goes, the amount of brain power we use is dependent on the degree of difficulty of the task at hand but in truth, human beings use 100% of our brain.
I hope this helps.