Question

how much broth/water (what isnt the original sample) would you include to * 1 point make 2 ml total of a 1:40 dilution?
50 ul
1950 ul
0.05 ul
19.5 ul
how much original sample should i include to make 2500 ul of a 1:20 * 1 point dilution?
50000 ul
125 ul
25 ul
2480 ul

Explanation:

Step1: Calculate volume of original sample for first question

For a 1:40 dilution, if the total volume $V_{total}=2\ mL = 2000\ \mu L$, and the ratio of original sample to total is $\frac{1}{40 + 1}=\frac{1}{41}$. The volume of the original sample $V_{sample}=\frac{1}{41}\times2000\ \mu L\approx 48.78\ \mu L$. The volume of broth/water $V_{diluent}=2000 - \frac{2000}{41}=2000\times(1 - \frac{1}{41})=2000\times\frac{40}{41}\approx1951.22\ \mu L\approx1950\ \mu L$.

Step2: Calculate volume of original sample for second question

For a 1:20 dilution, if the total volume $V_{total}=2500\ \mu L$, and the ratio of original sample to total is $\frac{1}{20 + 1}=\frac{1}{21}$. The volume of the original sample $V_{sample}=\frac{1}{21}\times2500\ \mu L\approx119.05\ \mu L\approx125\ \mu L$ (closest value among options).

Answer:

1. B. 1950 ul
2. B. 125 ul