So we know that the bacteria grows 3x as much every ten minutes. From this we know that it'll be 3 mm in 20 minute. Continuing the pattern, we can figure out that in 40 minutes the bacteria will be 81 mm.
The pattern: 10min - 3 20min - 9 30min - 27 40min - 81
10/8 can be simplified to 5/4, which is the same as 1.25 (divide numerator 5 by denominator 4).
So you have x>1.25.
All numbers greater than 1.25 are possible solutions.
If the question is how many shelves are needed, then the answer is 18 shelves.
