At Salisbury Central School, 60% of the students are 7th graders, and the
rest are 8th graders. Half of the 7th graders ride a bus to school, 20% of the
remaining 7th graders arrive by car, and the rest walk to school. A quarter of
the 8th graders ride a bus to school, a third of the remaining 8th graders arrive
by car, and the rest walk to school. What percent of Salisbury Central School
students walk to school?