Questions on concentration
To clarify, I've reworded your questions. Please comment if I changed the meaning:


Calculate the volume of pure water that needs to be added to dilute a  0.2M HCl solution with a volume of 250cm3 to a 0.12M HCl solution


The first step would be to work out how many moles of HCl is in the original solution. As you have stated

c = n / v

c = 0.2M

v = 250cm3 or 0.250L.

Therefore n = 0.05m

Then work out how much water is needed to make a 0.12M solution.

c = 0.12M

n = 0.05m

Therefore v = 0.42L

To work out how much water needs to be added, subtract the original 0.25L from 0.42L, which equals 0.17L


Calculate the mass of Na needed to react with 400cm3 of water to form a 0.15M NaOH solution.


As you haven't progressed much on this question, I'll give a few hints.

Write a balanced equation

Work out how many moles of NaOH is needed to make 0.15M solution

Work out how many moles of Na is needed to form that many moles of NaOH

Convert from moles to mass.

Hope this helps.