How would you prepare 100 ml of 0.4 M magnesium sulfate from stock solution of 2 M magnesium sulfate?

V1M1 = V2M2(x ml)(2 M) = (100 ml)(0.4 M)

x = 20 ml

Take 20 ml of 2 M magnesium sulfate solution and dilute to a final volume of 100 mls.