In a theatre, there are 20 seats in the front row and 30 rows were allotted. Each successive row contains two additional seats than its front row. How many seats are there in the last row?

In a theatre, there are 20 seats in the front row and 30 rows were allotted. Each successive row contains two additional seats than its front row. How many seats are there in the last row?
Solution :
Number of seats in 1st row  = 20
Number of seats in 2nd row  = 20 + 2  = 22
Number of seats in 3rd row  = 22 + 2  = 24
Total number of rows in theatre (n)  = 30
a = 20, d = 2 and n = 30
number of seats in last row be "tn" or "l"
n = [(l - a)/d] + 1
30  = [(l - 20)/2] + 1
(30 - 1)  = (l - 20)/2
29 (2)  = l - 20
l - 20  = 58
l  = 58 + 20
l  = 78
Hence the number of seats in the last row is 78.

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