A scientist wants to estimate the number of fish in a lake.
She catches 60 fish from the lake and marks them with a non-harmful dye.
The scientist then returns the fish to the lake.
The next day she catches another 30 fish and 4 of these are marked with the dye.
She estimates the total number of fish in the lake to be 450.
Which of the following assumptions has she made to work out her estimate?
Select all that apply.