A signal cos(2$\pi$f${_m}$t) modulates a carrier cos(2$\pi$f${_c}$t) using the double -sideband-with-carrier (DSDWC) scheme to yield a modulated signal cos(2$\pi$f${_c}$t) + 0.3 cos(2$\pi$f${_m}$t) cos2$\pi$f${_c}$t. The modulation index is $\_\_\_\_\_$ .(Answer should br rounded off to one decimal place)