The concentration of a pollutant bacteria c in a lake change

The concentration of a pollutant bacteria c in a lake changes according to

c= 75*e^-1.5t + 20*e^-.75t

determine the time required for the bacteria concentration to be reduced to 15 using

1) graphical method 0<=t<=5 seconds.(using matlab- need no solution-graph only)

2) matlab fzero

3) matlab solve (only show the real root)

Solution

t=0:0.1:5;
c= 75*exp(-1.5.*t) + 20*exp(-.75.*t);
plot(t,c)

result:

Columns 1 through 16

   95.0000   83.1080   72.7755   63.7924   55.9772   49.1733   43.2453   38.0764   33.5658   29.6261   26.1821   23.1684   20.5288   18.2144   16.1830   14.3980

Columns 17 through 32

   12.8277   11.4447   10.2252    9.1485    8.1966    7.3541    6.6072    5.9444    5.3553    4.8309    4.3636    3.9466    3.5738    3.2402    2.9412    2.6728

Columns 33 through 48

    2.4316    2.2145    2.0189    1.8424    1.6829    1.5385    1.4078    1.2893    1.1816    1.0838    0.9948    0.9137    0.8397    0.7722    0.7105    0.6541

Columns 49 through 51

    0.6025    0.5552    0.5118