Observe the following:

1 + 2 – 3 + 4 + 5 – 6 – 7 + 8 – 9 = –5


Change one ‘–’ sign as ‘ + ’ sign to get the sum 9.


We can observe that,


1 + 2 – 3 + 4 + 5 – 6 – 7 + 8 – 9


(1 + 2 – 3) + (4 + 5 – 9) – 6 – 7 + 8


0 + 0 + 2 – 7


2 – 7


If we change the sign of 7 from ‘–’ to ‘ + ’ it would become,


2 + 7


9


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