The first dilution is from 0.1mL -> 25mL, which is 1 -> 250 in the stock solution (just multiply both sides by 10). Then we add 1mL of the stock solution to 1mL of the enzyme solution, which is a 1 -> 2 dilution. so the final answer should be [E]x500 (you get 500 from 250x2 from both dilutions).
