Exponential growth

[Guest]

The bacteria in an 11 liter container double every four minutes. After 50 minutes, the container is full. How long did it take to fill 1/4 of the container?

How do I set this up to solve???

[shyamjayakannan]

go backward from the end. 4 min earlier, the container was half full and 4 min before that, it was half of half or one fourth full. so the answer is 50 - 8 = 42 min