The first question: the sum is 400, the average is 5, and the number is n, so it is 5×n = 400；; Quantity n=80

Question 2: According to the meaning of the question, the formula is: (a+4+2+5+3)÷5=b, and it is deduced that: A+14 = 5b; So a-5b=- 14.

Question 3: According to the meaning of the question: x1+x2+x3+...+x10 = 5×10 = 50; x 1 1+x 12+x 13+0....+x20=3× 10=30

x 1+x2+x3+....+x20=50+30=80

So the average of x 1, x2, x3, ... x20 is 80÷20=4.