Composite rate = [fixed rate + (2 x semiannual inflation rate) + (fixed rate x semiannual inflation rate)]
semiannual inflation rate = CPI March 2022 / CPI Sept 2021 - 1 = 287.504/274.310 - 1 = 4.81%
assume fixed rate 0%
composite rate = 0% + ( 2 x 4.81%) + (0 x 4.81%) = 9.62%