Answer:
4.17mL of the stock solution of ethidium bromide must be added in the 50mL agarose solution
Explanation:
You want to dilute the ethidium bromide from 6mg/mL to 0.5mg/mL, that is:
6mg/mL / 0.5mg/mL = 12 times the stock solution must be diluted.
As the final volume of the solution is 50mL, you must add:
50mL / 12 =
4.17mL of the stock solution of ethidium bromide must be added in the 50mL agarose solution