10 ml of a 12% solution from a 20% stock
We can use the relation: C1V1 = C2V2
where C1 and C2 are the concentrations of solutions and V1 & V2 are the volumes of respective solution.
Here, C1 = 12%, C2 = 20% and V1= 100 ml. We have to figure out how much of 20% solution is required to make a 12% solution.
Hence V2 = C1V1/C2 = (100x12)/20 = 60 ml
Therefore, we can use 60 ml of 20% stock solution and add 40 ml (=100-60 ml) of water and mix everything to obtain a 100 ml 12% solution.
Hope this helps.