Q.

Consider an A.P. of positive integers, whose sum of the first three terms is 54 and the sum of the first twenty terms lies between 1600 and 1800. Then its 11th term is :

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a

90

b

122

c

108

d

84

answer is C.

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Detailed Solution

S3 = 3a + 3d = 54
 a + d = 18
S20 = 10(2a + 19d)
 10(36 + 17d)
 1600 < 10(36 + 17d) < 1800
 160 < 36 + 17d < 180
 124 < 17d < 144
7517<d<8817
Common difference will be natural number
 d = 8  a = 10
 a11 = 10 + 10 × 8 = 90

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