My Math Forum Why principal is taken as 10 instead of 100?

 Elementary Math Fractions, Percentages, Word Problems, Equations, Inequations, Factorization, Expansion

 July 18th, 2019, 04:53 AM #1 Senior Member   Joined: Aug 2014 From: India Posts: 458 Thanks: 1 Why principal is taken as 10 instead of 100? If the S.I for 6 years be equal to 30% of the principal, it will be equal to the principal after how many years? Sol: $\displaystyle → 3 = 10*6*\large\frac{R}{100}$ $\displaystyle → R = 5%$ $\displaystyle → 10 = 10*T*\large\frac{5}{100}$ $\displaystyle → T = 20 yrs$ Why principal is taken as 10 instead of 100? Last edited by Ganesh Ujwal; July 18th, 2019 at 04:56 AM.
 July 18th, 2019, 06:23 AM #2 Senior Member   Joined: Jun 2019 From: USA Posts: 120 Thanks: 40 It doesn't really matter, since that is not necessarily the principal. The important thing is the 3 is 30 % of the 10. You could write it as 30 = 100*6*(R/100). More importantly, since simple interest is proportional to time, $\displaystyle \frac{I_1}{T_1} = \frac{I_2}{T_2} \rightarrow T_2 = (\frac{100 \%}{30 \%})(6~years) = 20~years$
 July 18th, 2019, 07:08 AM #3 Senior Member   Joined: Apr 2014 From: UK Posts: 953 Thanks: 340 I took the wordy approach: If we get 30% after 6 years, that's 30%/6years = 5% per year. 100%/5% per year is 20 years

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