Solution X is 15% sugar by volume, and Solution Y is 20% sugar by volume. 200 milliliters of Solution X is mixed with 400 milliliters of Solution Y to create a new mixture. If 100 milliliters of water evaporates from the new mixture. the resulting mixture will be what percent sugar by volume?
(A) 18
(B) 19
(C) 20
(D) 21
(E) 22

Please note that this is not an official GMAT question; it’s my attempt to create mid-range (500 to 650-level) GMAT-style questions for this forum.