AQA A-level Biology question

In humans, after the gametes join at fertilisation, every cell of the developing
embryo undergoes mitotic divisions before the embryo attaches to the uterus lining.
The first cell division takes 24 hours.
The subsequent divisions each take 8 hours.
After 3 days, the embryo has a total volume of 4.2 × 10–3 mm3
.
What is the mean volume of each cell after 3 days? Express your answer in
standard form.

Answer: 3.3 x 10–5 OR 3.28 x 10–5 OR 3.281 x 10–5
;;
1 mark for
Evidence of 128 (cells)
Correct numerical calculation but not in standard
form gains 1 mark (0.00003281 OR 0.0000328 OR
0.000033);
Im not sure how they have calculated the answer to this question
Original post by kit2247
In humans, after the gametes join at fertilisation, every cell of the developing
embryo undergoes mitotic divisions before the embryo attaches to the uterus lining.
The first cell division takes 24 hours.
The subsequent divisions each take 8 hours.
After 3 days, the embryo has a total volume of 4.2 × 10–3 mm3
.
What is the mean volume of each cell after 3 days? Express your answer in
standard form.

Answer: 3.3 x 10–5 OR 3.28 x 10–5 OR 3.281 x 10–5
;;
1 mark for
Evidence of 128 (cells)
Correct numerical calculation but not in standard
form gains 1 mark (0.00003281 OR 0.0000328 OR
0.000033);
Im not sure how they have calculated the answer to this question

right so the 1st division is 24hrs which is a day. so in 3 days u get 72hrs. 24hrs out of the 72 are solely for the first division so 72-24=48hrs these 48 hrs are for the subsequent divisions which takes 8hrs so 48/8hrs gives u 6 more divisions so a total of 7 divisions. u can draw the diagrams where 1 turns to 2 the 2 gives 4 and so on or mathematically u just make it 2 to the power of 7 which gives 128 cells in total. then u get mean by taking the volume of 4.4*10^-3 divide by 128 gives the 3.28*10^-5