The Number Rainbow - 12/04/2013

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The Number Rainbow - 12/04/2013

Postby showmyiq » Fri Apr 12, 2013 7:18 am

Which number is not like the others in this number rainbow?
Why?

15
28
45
56
78
91
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Re: The Number Rainbow - 12/04/2013

Postby Gusto » Wed Apr 17, 2013 2:56 pm

78
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Re: The Number Rainbow - 12/04/2013

Postby showmyiq » Wed Apr 17, 2013 7:32 pm

Can you please explain how did you reach that answer?
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Re: The Number Rainbow - 12/04/2013

Postby showmyiq » Fri Apr 19, 2013 6:39 am

Since I didn’t received an explanation of that answer I highlighted as incorrect.
The correct answer is 56 and it was successfully solved by Sergey Sokolov via facebook.

The pattern here is adding all the numbers up to N. This are called the triangular numbers and I have already published a sections with them.

1
1+2 = 3
1+2+3 = 6
1+2+3+4 = 10
1+2+3+4+5 = 15
1+2+3+4+5+6 = 21
1+2+3+4+5+6+7 = 28
1+2+3+4+5+6+7+8 = 36
1+2+3+4+5+6+7+8+9=45
1+2+3+4+5+6+7+8+9+10=55
1+2+3+4+5+6+7+8+9+10+11=66
1+2+3+4+5+6+7+8+9+10+11+12=78
1+2+3+4+5+6+7+8+9+10+11+12+13=91

Therefore 56 is the wrong number in this number set! It’s not triangular number :)
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