The correct probability is 85/103. Her errors were that she only took the number of 7th grade students that read 100 pages per week; she did not include the number of students that read more than that, and she wrote the denominator of the probability as the number of pages read instead of the total number of seventh grade students.
32 seventh grade students read 100 pages per week. 53 students read 150 pages per week. This is out of 18+32+53=103 seventh grade students. This gives us 32+53=85/103.
