The simple interest for 9 years on a principal is $\frac{3}{5}$ of the principal. What is the rate of interest per annum?

$6\frac{2}{3}\%$

Simple Interest = \(\frac{3 }{5 }\) × Principal

Simple Interest = \(\frac{Principal ×Rate × Time }{100}\)

\(\frac{3 }{5 }\) × Principal = \(\frac{Principal ×Rate × 9 }{100}\)

\(\frac{3 }{5 }\) = \(\frac{Rate × 9 }{100}\)

Rate = \(\frac{20 }{3}\)%

= 6\(\frac{2 }{3}\)%