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Let 3, a, b, c be in A.P. and 3, a-1, b+1, c+9 be in G.P. Then, the arithmetic mean of a, b and c is :
(4) 11
A.P. with common difference d: a=3+d, b=3+2d, c=3+3d
G.P.: 3, 2+d, 4+2d, 12+3d
(2+d)2=3(4+2d)
d2-2d-8=0 d=4 or d=-2
∵ d=-2 gives 3, 0, 0, 6 (not a G.P.)
d=4, G.P.: 3, 6, 12, 24
a=7, b=11, c=15
∴ A.M. = 7+11+153=11
Answer: (4)
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