13.312. When unloading a barge, four lifting cranes of the same power initially worked for 2 hours. Then, two additional cranes of lesser but equal power were put into operation. After this, it took another 3 hours to complete the unloading. If all these cranes had started working simultaneously, the unloading would have been completed in 4.5 hours. If one crane of greater power and one crane of lesser power worked together, how long would it take them to unload the barge?