If 45 ml of h2o are required to mix 100 g of casting plaster, how many milliliters would you need for 250 g of plaster ?

Answer is 112.5 mL

You add 45 and another 45 to get 90 mL for 200g, and divide 45 by 2 to get 22.5 for the 50g left and you add all up 90+22.5=112.5