∴ The proportion of the D-class population is 4%.

∵4%×50=2,

The number of students in Grade D is 2;

(2)∵A-level students 30%×50= 15,

The number of B-level students is 48%×50=24.

The number of students in Grade C is 18%×50=9.

The number of students in Grade D is 4%×50=2.

The median of ∴ falls in Grade B;

(3) Number of qualified personnel =800×(30%+48%+ 18%)=768,

About 768 people passed the exam.