To simplify the argument, let assume total number of seniors in 1997 is 100:
In 1997, 20% seniors did not pass the exam -> 80% pass -> it is 80 pass against 100 in total
In 1998, number of passed senior decreased 10% (of 80) -> it is 72 pass against "X" in total
=> If ratio of passed senior in 1998 equals to that in 1997, then X = 72/80% =90
If X > 90, then 1998's ratio <80%
If X <90, then 1998's ratio > 80%
We have to eliminate all the answer choices
...
In 1997, 20% seniors did not pass the exam -> 80% pass -> it is 80 pass against 100 in total
In 1998, number of passed senior decreased 10% (of 80) -> it is 72 pass against "X" in total
=> If ratio of passed senior in 1998 equals to that in 1997, then X = 72/80% =90
If X > 90, then 1998's ratio <80%
If X <90, then 1998's ratio > 80%
We have to eliminate all the answer choices
...
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