Sum data validation

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Thursday November 13, 2014
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November 13, 2014
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someone help me to figure out this problem.
example :

A1 + B1 = C1
A2 + B2 = C2
.
.
.
.
A9 + B9 = C9

If A1 ~ A9 data changed using dropdown list.
Result for C also changed.

How to use data validation option if I want C column total not exceed for some figure.