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If you mean by give the sum of 5+10+20+40, then the answer is 75. But if the question is find the sum of infinity, then : 
              S_{n}= \frac{a_{1}}{1-r}
              S_{n}= \frac{5}{1-2}
              S_{n}= \frac{5}{-1}   OR    -5
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