2018 AQA AS paper 2 question 8.2

Could someone kindly help me out with this?

"Using an optical microscope, the student determined there were 15 cells in 0.004 mm box 3 of the 1 in 1000 dilution of the culture. Calculate the number of cells in 1cm3 of undiluted liquid culture."

My workings:
1cm = 10mm
10 / 0.004 = 2500
2500 x 15 = 37 500
37 500 x 1000 = 37 500 000
37 500 000 = 3.75 x 10^7

Correct answer according to mark scheme:
3.75 x 10^9

Where do I get the extra two zeros?!

Reply 1

HarryH01

3

You worked it out per mm^3 and its asking per cm^3

Original post by Kinyonga

Could someone kindly help me out with this?

"Using an optical microscope, the student determined there were 15 cells in 0.004 mm box 3 of the 1 in 1000 dilution of the culture. Calculate the number of cells in 1cm3 of undiluted liquid culture."

My workings:
1cm = 10mm
10 / 0.004 = 2500
2500 x 15 = 37 500
37 500 x 1000 = 37 500 000
37 500 000 = 3.75 x 10^7

Correct answer according to mark scheme:
3.75 x 10^9

Where do I get the extra two zeros?!

Reply 2

Kinyonga

OP

20

Original post by HarryH01

You worked it out per mm^3 and its asking per cm^3

I worked it out for 10mm, which, unless my brain is on the blink, is equivalent to 1cm. So surely it doesn't make any difference.
And either way, that would just make me lose zeros, not gain them.

Reply 3

HarryH01

3

0.004 mm^3 = 0.000004 cm^3
then 1/0.000004 = 250000
250000 x 15 x 1000 = 3.75x10^9

its a sneeky conversion because its mm cubed, so it actually is quite different :smile:

Reply 4

Kinyonga

OP

20

Original post by HarryH01

0.004 mm^3 = 0.000004 cm^3
then 1/0.000004 = 250000
250000 x 15 x 1000 = 3.75x10^9

its a sneeky conversion because its mm cubed, so it actually is quite different :smile: