(1) According to the frequency represented by the rectangular area of the frequency distribution histogram, a rectangular area with a score of 50-70 points is obtained.

(2) Find out the frequency of the third group, and then find out what you want according to the total number of students in three grades = frequency frequency;

(3) First, find the frequency of 80- 100, and then use frequency = total number of students × frequency to find the number of students with 80- 100.

Solution: The scoring frequency of (1)50-70 points is: 0.03 *10+0.04 *10 = 0.7 … 4 points.

(2) The frequency of the third group is 0.015 *10 = 0.15.

The total number of students in these three grades (total number = frequency/frequency) is:15/0.15 =100 (person)? ... eight points

(3) The frequency of the score of 80-100 is 0.01*10+0.005 *10 = 0.15.

So the number of people with 80- 100 is:100 * 0.15 =15 (people)? ... 12 point