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