My tutor is growing bacteria. She starting off with a mass of 150 grams of bacteria at 12 pm on tuesday, than every hour the mass of bacteria is growing by 15%, but my tutor wants to end up with 3 grams of bacteria.

Should I use a computer or calculator on my level of precision and why should I use it, and using the appropriate device how long will it take for the mass of bacteria to reach 3 grams, and waht is the exact date and time. I am having some serious trouble with this! thankyou