sorry i mixed up the negative sign in the last answer and completely messed up. lemme do it again, correctly.Let the number be xthen , our numbers are:(-1+x), (1 +x) , (5+x)thus, (-1+x)(5+x) = (1+x)^2 ( this is how g.s. work)or, -5-x + 5x + x^2 = 1 + 2x + x^2or, -5 + 4x = 1 + 2xor, 4x - 2x = 1 + 5or, 2x = 6thus, x = 3the number required is 3.just to make sure, u should check and not make mistake as i did earlier. =D1st number = ( -1 + 3 ) = 22nd number = ( 1 + 3) = 43rd number = ( 5 + 3 ) = 8they are in a geometric sequence with common ratio 2. thus, the answer we got i.e. 3 is correct.