There are 100 sheets of paper in the printer, and the number of sheets of paper, p, left after ,t, minutes of printing is given by the function p(t) = -8t + 100.
how many minutes would it take the printer to use all 100 sheets of paper?

P(t)=number of sheet of paper after t minutes.
p(t) will have to be equal to 0, wnen the printer use  all 100 sheets of paper;
Therefore;
if p(t)=0
Then:

-8t+100=0

We solve this equation.
-8t+100=0
8t=100
t=100/8
t=12.5

the printer prints 100 sheets of paper in 12.5 minutes.