I've been struggling with these two exponential growth and decay problems.

In days, an unknown radioactive substance decay to percent of its size.

(a) What is the half life of this substance?

(b) How long will it take for a sample of 100mg to decay to mg?

Also,

A cell of some bacteria divides into two cells every 10 minutes.The initial population is 4 bacteria.

(a) Find the population after t hours

(b) Find the population after 9 hours.

(c) When will the population reach 36?