Original sample (7mL): transfer 0.2mL from the original into 3.8mL blank and pla

Question

Original sample (7mL): transfer 0.2mL from the original into 3.8mL blank and plate 0.2mL
Plate A has 450 colonies
Transfer 0.1mL from the 3.8mL dilution blank into 9.9mL dilution blank and plate 0.2mL
Plate B has 80 colonies
Transfer 1mL from the 9.9mL dilution blank into 5mL dilution blank and plate 0.2mL
Plate C has 2 colonies.

Can someone please explain how to get to these answers I am confused how the whole dilution and concentration works. Thank you

Original sample (7mL): transfer 0.2mL from the original into 3.8mL blank and plate 0.2mL Transfer 0.1mL from the 3.8mL dilution blank into 9.9mL dilution blank and plate 0.2mL Transfer lmL from the 9.9mL dilution blank into 5mL dilution blank and plate 0.2mL Plate A has 450 colonies Plate B has 80 colonies Plate C has 2 colonies 0.2mL 0.1mL mL 7mL 3.8mL 9.9mL 5mL 0.2mL 0.2mL 0.2mL 450 colonies 80 colonies 2 colonies What is the concentration in the 3.8mL blank? a. 450/0.2 b. 450/0.2 x 20 c. 450/0.2 x 19 d. 80/0.2 x 100 e. 80/0.2 x 99 What is the concentration in the original? a. 450/0.2 x 19 b. 450/0.2 x 20 c. 80/0.2 x 100 x 20 d. e. 80/0.2 x 99 x 19 80/0.2 x 10 What is the concentration in the 5mL blank? a. 2/0.2 or 10/mL b. 10/0.2 x 5 c. 2/0.2 x 1/5 80/0.2 x 1/5 80/0.2 x 1/6 d. e.

Explanation / Answer

1) Total colonies in 9.9 ml blank are 80, and that are present in 0.2 ml of culture.

So, the concentration of 9.9 ml blank would be 80 / 0.2

9.9 ml blank is 100 times dilution from the 3.8 ml blank (0.1 ml + 9.9 ml) = 10 ml; 10 / 0.1 = 100

So, the concentration of 3.8 ml blank is 80 / 0.2 X 100

2)  The concentration of 3.8 ml blank is 80 / 0.2 X 100

3.8 ml blank is 20 times dilution from the original sample.

3.8 ml + 0.2 ml = 4 ml; 4 / 0.2 = 20 times

Therefore, the concentration of 9.9 ml blank is 80 / 0.2 X 100 X 20

3) The concentration of 9.9 ml blank is 80 / 0.2

5 ml blank is 6 times dilution from the 9.9 ml blank (1 : 5).

Therefore, the concentration of 5 ml blank is 80 / 0.2 X 1 / 6

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