how much water would you need to add to 20.0 mL of 2.0 M NaCl to make a 0.10 M solution?
try the formula
c1V1 = c2V2
(where c1 is the initial concentration, V1 is the initial volume, c2 is the diluted concentration and V2 is volume you are trying to find)
when you have found V2 - remember V2 is the final volume including the original 20mL - so to work out how much to add you need to subtract the 20mL from V2.
Another way to look at this, is you are diluting it 20 times (2/.1), so add nineteen parts water to one part original solution.
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