J.R. S. answered 07/05/18
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Ph.D. in Biochemistry--University Professor--Chemistry Tutor
V1C1 = V2C2 in chemistry terminology
V1 = initial volume = ?
C1 = initial concentration = 20%
V2 = final volume = 15 ml
C2 = final concentration = 0.9%
(V1)(20%) = (15 ml)(0.9%)
V1 = (15)(0.9)/20 = 0.0.675 ml of the stock solution
15 ml - 0.675 ml = 14.325 ml of diluent
NOTE: These values are not adjusted to the correct number of significant figures as I'm not sure you are being asked to use such a correction.