Since both the channels start transmitting at the same time, collision will occur when the signal is halfway so , collision occurs at 112.5 bit time and then it takes 112.5 bit time again for the collision information to reach respective nodes. Therefore, collision is detected at 225 bit time.
Bit time can be converted to secs as : $\frac{225}{10^7} sec$ = 22.5 μs
Now they will transmit 50 bit jam signal. To transmit 50 bits we need $\frac{50}{10^7} sec$ = 5 μs
The jamming signal again takes 225 bit time to propagate to the other end. i.e 22.5 μs
Therefore total time after which the channel will be clear = 22.5 + 5 + 22.5 = 50 μs