Your tool of choice here is the dilution factor,
- The ratio that exists between the volume of the diluted solution and the volume of the stock solution
- The ratio that exists between the concentration of the stock solution and the concentration of the diluted solution
So for any dilution, you have
$"DF" = color(white)(overbrace(color(black)(V_"diluted"/V_"stock"))^(color(blue)("volume ratio: diluted/stock"))) = color(white)(overbrace(color(black)(c_"stock"/c_"diluted"))^(color(blue)("concentration ratio: stock/diluted")))$
Now, you know that you're performing a
$2:25 = 1:color(blue)(12.5)$
dilution that will have
$"DF" = color(blue)(12.5)$
This tells you that the stock solution was
If you want the volume of the diluted solution to be equal to
$V_"stock" = V_"diluted"/"DF"$
Plug in your values to find
$V_"stock" = "200 mL"/color(blue)(12.5) = "16 mL"$
I'll leave the answer rounded to two , but keep in mind that you only have one significant figure for the volume of the diluted solution.
So, you can perform this dilution by taking