what is the volume that must be added to dillute 20.0 ml of 0.1M Hcl to 0.02M Hcl

mL1 x M1 = mL2 x M2

20 x 0.1 = mL2 x 0.02
Solve for mL2 which gives you the FINAL volume. Subtract the final - initial to find amount to be added. Technically, that will not be right since volumes are not additive but for dilute solutions like this the difference is minuscule.