# How to ask a time interval elapsed since a certain point time?

Consider there are 100 bacteria at 12:00. Each bacterium divides into 2 every 10 minutes, for example. The process of dividing into 2 is called binary fission.

• The first binary fission occurs 10 minutes after 11:00.
• The second binary fission occurs 20 minutes after 11:00.

# Question

How to ask the bold words? My attempt is as follows, but I am not sure whether it is correct or not.

Find the elapsed time since 11:00 at which the n-th binary fission occurs.

How many minutes after 11:00 does the n-th binary fission occur?

The use of "n-th" means that the context must be a maths test. So if you can use mathematical notation you can be less ambiguous but more wordy:

Let t(n) be the time in minutes after 11:00 at which the n-th binary fission occurs.

Find an expression for t(n) in terms of n.

Possibly:

If binary fission (of the bacteria) first occurs at 11.10 and repeats every ten minutes, determine the elapsed time before the nth division.

• The elapsed time before the first division is zero? Commented Feb 18, 2021 at 22:58
• Hmmm! How about? If the binary division (of x bacteria) begins at 11.00 and repeats every 10 minutes, determine the elapsed time before the nth division. (However, this overlooks the 100 bacteria at 12.00, which would be impossible if the bacteria concerned had begun splitting at 11.00. Did you mean 100 at 11.00?) Commented Feb 19, 2021 at 0:18