I think i should be 240 min
let CA capacity of Amir, CB capacity of Birju, CC capacity of Charles.
now Capacity=Work/Time
then form the question we can write
CA+CB=5000/20=500/2 => 2CA+2CB=500...............1
CB+CC=5000/40=250/2 => 2CB+2CC=250.................2
CC+CA=5000/30=500/3 => 3CC+3CA=500.................3
Now Solving the equation 1,2,3 for CC(As we need to answer about charles) we get CC=125 / 6
Again CC=work to be done by charles/Time take to do the work (t)
125/6=5000/t
t = (5000*6)/125
= 240 Min
hope it helps