(I am a mom trying to help my child had Chem years ago if I can see how this is doNE I can explain it to them)
You have 70.0mL of a .400M stock solution that must be diluted to .100M. How much water should I add?
Use the dilution formula.
mL1 x M1 = mL2 x M2
70 x 0.4 = mL2 x 0.100
mL2 = 70 x 0.4/0.100 = 280 mL.
So the FINAL volume must be 280 mL.
ASSUMING the volumes are additive (technically they are not but at these concentrations there will not be a measurable difference) so you already have 70 mL you will need to add 280-70 = 210 mL.
I like the dilution formula for it works no matter how the problem is stated. However, another way to do this is to say you want to dilute from 0.4 to 0.1 which is a factor of 4. So 4 x 70 = 280 final volume.