For this specific example, where the final volume is 50 mL, it may help to think about doing the dilution twice so that you end up with 100 mL total volume.

For this type of problem in general, I would search wikipedia for "dilution equation." When using that equation you can use any type of concentration (molarity, grams/L, percent by volume, etc.), but for both concentration and for volume make sure to use consistent units on both right and left sides of the equation.

If neither of these methods works out right for you, then please post your attempts/results here and I am sure you will get plenty of help with it.