The half-life of cobalt-57 is 270 days. A hospital buys 30mg of this substance.

a) Write a function that describes how much of the substance, 'M' mg, will remain after 'n' days. [2C]

b) How much of the substance will remain after 2 years? Recall there are 365 days in one year. [2T]

c) How long will it be until only 10% of the substance remains?