.
60 minutes comprise = = 7.5 doubling periods.
THEREFORE,
450 = ,
where x is the starting number of bacteria.
From this equation,
x = = use your calculator = 2.49 bacteria.
You may round it as you want.
I prefer to round to the closest greater integer number, which is 3.
Do not accept any other answer/solution,
and ignore the post by @josgarithmetic, which is INCORRECT and may confuse you.
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For bacteria growth problems, see the lesson
- Bacteria growth problems
in this site.
Also, you have this free of charge online textbook in ALGEBRA-I in this site
- ALGEBRA-I - YOUR ONLINE TEXTBOOK.
The referred lessons are the part of this online textbook under the topic "Logarithms".
Save the link to this online textbook together with its description
Free of charge online textbook in ALGEBRA-I
https://www.algebra.com/algebra/homework/quadratic/lessons/ALGEBRA-I-YOUR-ONLINE-TEXTBOOK.lesson
to your archive and use it when it is needed.