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প্রশ্নানুসারে, \(20000\left(1+\cfrac{r}{100}\right)^3=26620\)
বা, \(\left(1+\cfrac{r}{100}\right)^3=\cfrac{26620}{20000}=\cfrac{1331}{1000}=\left(\cfrac{11}{10}\right)^3\)
বা, \(1+\cfrac{r}{100}=\cfrac{11}{10}\)
বা, \(\cfrac{r}{100}=\cfrac{11}{10}-1=\cfrac{11-10}{10}=\cfrac{1}{10}\)
বা, \(r=\cfrac{100}{10}=10\)

\(\therefore r\) এর মান \(10\)