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Question
There are 25 rows of seats in an auditorium. The first row is of 20 seats, the second of 22 seats, the third of 24 seats, and so on. How many chairs are there in the 21st row ?
Solution
The arrangement of chairs is 20, 22, 24, 26, ..........
Which is an A. P.
Here, a = 20, d = 2. We want to find t21.
tn = a + (n-1)d
∴ t21 = 20 + (21-1) × 2
= 20 + 40
= 60
∴ There are 60 chairs in the 21st row.
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