chemical kinetics!

In any reaction there is no increase or decrease in moles, so A+B+C+D = constant

dA/dt + dB/dt + dC/dt + dD/dt = 0

also, given : dB/dt = -0.4(dA/dt) and dC/dt = -0.5(dA/dt)

from above three equations : dD/dt = -0.1(dA/dt)

rate of formation of D is 10% rate of decay of A

so partial half life wrt conversion of D = overall half life / 0.1 = 300 hours