Answer:
We have the equation: 
P(t) = 100 - 10*t 
where t is the number of hours. 
at t = 0, we have: 
P(0) = 100 - 0 = 100 
so at the initial time, we have 100% of the battery power. 
Now with this equation we can find the numbers of hours that we have left in our phone, this is finding the time t at which the power left is equal to zero: 
P(t) = 0 = 100 - 10*t 
10*t = 100 
t = 100/10 = 10 
So the battery power will last for 10 hours.