18. (6 points) Pour 100 grams of 40% sugar water into $a$ grams of 20% sugar water, resulting in 25% sugar water, then $a=$ $\qquad$
【Answer】Solution: According to the problem, we have:
Based on the cross method of concentration:

The ratio of the difference in concentration is equal to the ratio of the solution's mass
That is, 1: $3=100: a$, so $a=300$ grams
Therefore, the answer is: 300