To make 30 L of 5% formalin solution from 100% stock solution, proceed as follows:
You can use the formula V1C1 = V2C2 where V1 and V2 are volumes of initial and final solutions and C1 and C2 are concentrations (%) of initial and final solutions. Thus, ...
(x L)(100%) = (30 L)(5%)
100x = 150
x = 150/100
x = 1.5
So, you need 1.5 L of the 100 % stock diluted to a final volume of 30 L, which means you must add 28.5 L of diluent. (Note: This assumes the volumes are additive)