If the chairs in an auditorium are arranged in an arithmetic sequence 5 th row contains 31 chair and 16 th row contains 75 chairs then find the total number of chairs if there exists 20 rows

Answer 1

11 differences ... 75 - 31 = 44 ... 44 / 11 = 4

difference between adjacent rows is 4

find first row and last row ... average them ... multiply by half the number of rows

Answer 2

Why did you post this again after you received a good answer?

https://www.jiskha.com/display.cgi?id=1532539941

Answer 3

11 differences ... 75 - 31 = 44 ... 44 / 11 = 4

difference between adjacent rows is 4

find first row and last row ... add them ... multiply by half the number of rows

Answer 4

disregard my 1st response

Answer 5

Just use your formulas:

5 th row contains 31 ---> a + 4d = 31
16 th row contains 75 --> a + 15d = 75
subtract them:
11d = 44
d = 4
in a+4d = 31
a + 16 = 31
a = 15

so sum of 20 rows = (20/2)(30 + 19(4)) = ....