Answer: the correct answer is Wisconsin gains 300, Illinois loses 100.
Explanation:
Wisconsin won't clean the lake so we have to leave it out of the calculation.
Now Illinois is the one that spends 700 but receives just 600 in benefits.
700-600= 100 ( Illinois loss)
Out of the 900 generated for cleaning the lake, Michigan gets 300, because Wiconsin receives 600, that is,
900-600 =300 ( Wisconsin gain) (*Remember Wisconsin hasn't spent any single penny because it hasn't clean the lake).