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in Arithmetic Progression by (43.8k points)
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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 seats are there in the 21st row ?

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Correct Answer - There are 60 seats in the 21st row.
20 seats in the first row, 22 seats in the second row, 24 seats in the third row,….
20,22,24,…. is a sequence
The number of seats increasing inconsecutive rows
`= 24 -22 = 22-20=2`
`:. D= 2 `
`:. 20, 22, 24,.. ` is an A.P.
Here, a = 20
We have to find the number of seats in the 21st row, i.e., `t_(21)`.
`t_(n) = a+ ( n -1)d`
`:. t_(21) = 20 + ( 21-1) xx 2 `
` = 20 + 20 xx 2 `
`= 20 + 40`
`:. t_(21) = 60`
`:. t_(21) = 60`

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