I think you are dealing with a volume-to-volume percent concentration ("v/v%"v/v%), which is defined as the volume of the solute divided by the total volume of the solution and multiplied by 100%. This is mathematically described as
"v/v%" = ("volume of solute")/("total volume of solution") * 100%v/v%=volume of solutetotal volume of solution⋅100%
So, in your case, you have a volume of solute equal to 50muL50μL and a volume of solvent, NOT of total solution, equal to 200muL200μL.
You add the solute to the solvent and get a total volume of solution of
5050 muμ"L" + 200L+200 muμ"L" = 250L=250 muμ "L"L, which means that your "v/v"v/v percent concentration is
"v/v%" = (50muL)/(250muL) * 100%= 20%v/v%=50μL250μL⋅100%=20%
So, essentially, you have dilluted the original 50muL50μL sample to 1/5th of its original concentration:
C_("initial") * 50 muL = C_("final") * 250 muL => C_("final") = C_("initial") * (50muL)/(250muL)Cinitial⋅50μL=Cfinal⋅250μL⇒Cfinal=Cinitial⋅50μL250μL
C_("final") = C_("initial")/5Cfinal=Cinitial5.