Interesting exponential/logarithm example

One of the more budding students in my class derived this example question, I think his dad works in a hostpital, and its based on actual paperwork on MRSA in hospitals...anyway, If anyone can give me a hand as Im useless with exponentials..

- Hostpital rooms can be decontaminated of MRSA if sprayed with hydrogen peroxide, however this is harmful to people if there's prolonged exposure, so we must know its decay rate.

- A room was sprayed with vapour containing hydrogen peroxide, with a concentration of 22.80 mmoles/litre, and the level of this peroxide, ct, was measured over intervals:

t (min)----------0--------5-------10-------20------

Ct (mmoles/ltr) 22.80 -- 17.60 -- 13.80 -- 8.25

As far as I can conclude, the decay equation is ct = coe^-λt

a) how do you tranform the equation so the straight line of y=mx+c can be plotted and how are y, m and c related to ct, co, and λ