I think you are dealing with a volume-to-volume percent concentration (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 solutetotal volume of solution⋅100%
So, in your case, you have a volume of solute equal to 50μL and a volume of solvent, NOT of total solution, equal to 200μL.
You add the solute to the solvent and get a total volume of solution of
50 μL+200 μL=250 μ L, which means that your v/v percent concentration is
v/v%=50μL250μL⋅100%=20%
So, essentially, you have dilluted the original 50μL sample to 1/5th of its original concentration:
Cinitial⋅50μL=Cfinal⋅250μL⇒Cfinal=Cinitial⋅50μL250μL
Cfinal=Cinitial5.