If you start with 150 grams and it grows by 15% per hour, it's going to get bigger and bigger and will never shrink to 3 grams, so I think you may have used the wrong values. In general the amount of bacteris is :
where is the starting amount, 'i' is the growth rate per period, and 'n' is the number of periods. If you start with 150 grams and it grows 15% per hour then after 8 hours you have [tex[ 150(1+0.15)^8 = 458.8 grams.
For the inverse problem, if you are given a value for 'A' and want to figure out how many periods it takes to grow to that amount you can rearrange the above formula:
It doesn't matter what base you use for the logarithm - could be log base 10 or natural logs, whatever you prefer, just be consistent. For example to grow from 150 to 1000 grams takes: