The C.I. on a certain sum for 2 years is Rs. 410 and the S.I. is Rs. 400 at same rate for same time. Find the principal.

$SIfor2years=400$

$SIfor1year=200$

$CIfor1year=200$

$GivenCIfor2years=410$

$CIfor{2}^{nd}year=210$

$That\mathrm{int}erestin200for1year=210-200=10$

$I=\frac{PTR}{100}$

$10=\frac{200\times P\times R}{100}$

$R=5\%$

$\frac{PTR}{100}=400\Rightarrow \frac{P\times 2\times 5}{100}=400$

$P=4000$