The difference between simple interest and compound interest of a certain sum of money at 10% per annum for 1 year is Rs.1800. Then the sum is: (Interest being compound half yearly)

Rs.720000

Simple interest ⇒ 10% 

Compounded interest ⇒ 5% + 5% + \(\frac{5 × 5}{100}\)% ⇒ 10.25% 

Diff. b/w C.I & S.I. for two years ⇒ 10.25% - 10% ⇒ 0.25%

ATQ,

0.25% of P = 1800

1% of P = \(\frac{1800}{.25}\)

100% of P = \(\frac{1800}{25}\) × 10000

Principal = Rs.720000