biology aslevel aqa dilution

The student diluted 3 cm3 of culture 1 with 12 cm3 of water. She observed a sample of this diluted mixture using an optical microscope and counted 24 cells in 0.000 25 mm3 of the diluted mixture.
Use this information to calculate the number of cells per mm3 in undiluted culture 1
ans is 480 000

To find the number of cells per mm² in the undiluted culture 1, we can use the concept of dilution.
The dilution factor is the ratio of the volume of the original solution to the volume of the final solution. Here, the dilution factor is:
Dilution Factor = Volume of Original Solution / Volume of Final Solution
= 3 cm³ / (3 cm³ + 12 cm³)
= 3 cm³ / 15 cm³
= 1/5
So, each cell in the diluted mixture represents 1/5 of a cell in the undiluted culture.
Now, if there are 24 cells counted in the diluted mixture, this corresponds to:
24 cells * 5 = 120 cells in the undiluted culture.
Given that the count was done in 0.00025 mm², to find the number of cells per mm²:
Number of cells per mm² = (120 cells) / (0.00025 mm²)
= 480,000 cells/mm²

why did u multiply 24 with 5? sorry i'm a bit slow lol