A source emits light of wavelengths 555 nm and 600 nm . The radiant...

The total radiant flux is defined as the total power of electromagnetic radiation emitted by the source. It is simply the sum of the radiant fluxes of all the individual wavelength components.

Given:
Radiant flux of the $555 \text{ nm}$ part $= 40 \text{ W}$
Radiant flux of the $600 \text{ nm}$ part $= 30 \text{ W}$

Total radiant flux $= 40 \text{ W} + 30 \text{ W} = 70 \text{ W}$

The relative luminosity is required only for calculating the luminous flux and is not used in finding the total radiant flux.