Posted by fran on Sunday, September 15, 2013 at 1:06am.
The integer 90can be expressed as the sum of z consecutive integers. The value of z could be any of the following except
A) 3
B) 4
C) 5
D) 6
E) 9

maths  Graham, Sunday, September 15, 2013 at 6:29am
If the first integer is n+1, the consecutive integers are: n+2, ...n+z.
If 90 is the sum of these z consecutive integers then...
90 = sum{i=1 to z} n+i
90 = zn + sum{i=1 to z} i
90 = zn + z(z+1)/2
.: n = (180z(z+1))/2z
Testing the given values of z:
A) z=3 => n = (1803*4)/6
B) z=4 => n = (1804*5)/8
C) z=5 => n = (1805*6)/10
D) z=6 => n = (1806*7)/12
E) z=9 => n = (1809*10)/18
One of these 'n' is not an integer.
One of these 'n' does not belong.
Can you tell me which one?

mathT  Steve, Sunday, September 15, 2013 at 6:33am
(D)6
6x + 5*6/2 = 90
6x + 15 = 90
6x = 75
x is not an integer
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