Exponential growth percentage conversion

Question

The question states:

If bacteria is doubling in size every 20 minutes, at what % rate is the number of bacteria increasing every hour?

So firstly using the rule of 70, I approximated that the percentage growth rate is 3.5%. This is however every minute. How do I convert it to every hour, should I just multiply it by 60? Thank you!

Answer 1

If bacteria is doubling in size every 20 minutes, what about the size after 40 minutes?, and after 60 minutes?

Hope this helps.

Answer 2

Basically, after every $20$ minutes, the bacteria would double in size. This would mean that since $60=20\times3$, so the size of the bacteria after the $60$ minutes would be $2^3=8$ times the size of the bacteria before the $60$ minutes. But the question asks for the percentage, so it should probably be $800-100=700\%$, because it asks for the growth, and not the entire comparison (if you know what I mean).