How much water should be added to 30 l of a 40 acid solution to reduce it to a 30 solution?

To solve this we use the equation, 

M1V1 = M2V2

where M1 is the concentration of the stock solution, V1 is the volume of the stock solution, M2 is the concentration of the new solution and V2 is its volume.

M1V1 = M2V2

40% x 30 L = 30% x V2

V2 = 40 L 

Therefore, you will need to have 30 mL of the 40% acid solution and 10 mL of distilled water. In mixing the two liquids, you should remember that the order of mixing would be acid to water. So, you use a 40 mL volumetric flask . Put small amount of distilled water and add the 30 mL of HCl solution. Lastly, dilute with distilled water up to the mark.