(1) First, get the number of B speakers according to the histogram, then get the number of A speakers according to the number of B speakers: 1: 5, and then get the sample size of the survey by using the department statistics chart;

(2) The number of people in Group C = the percentage of the total number of people ×C, and then draw a picture according to the number of people;

(3) According to the statistical table, it is found that the number of people who speak in class at least 15 times a day is in three groups: D, E and F. Find out the percentage of people in group B, and then use 1-4%-40%-20% to get the percentage of people in three groups: D, E and F. The answer can be calculated by sample estimation.

Solution: (1) ∵ There are 10 people in group B, and the number of speakers in group A: the number of speakers in group B = 1: 5.

∴: The number of speakers in Group A is 2.

The sample size of this survey is: 2 ÷ 4% = 50;

(2) The number of people in Group C is 50×40%=20; I won't say much about histogram. .

(3) The percentage of speakers in Group B:10/50x100% = 20%, and the number of speakers who speak at least 15 times a day in the whole grade is 250x (1-4%-40%-20%) = 90.