Let us calculate the speed of each conveyor belt. The bigger one does 300 pounds in 10 minutes, hence its speed is 300 pounds/ 10 minutes=30 pounds/min. The smaller one needs 30 minutes, hence its speed is 300 pounds/ 30 minutes=10 pounds/min. We see that the total speed of both machines if they work together is 30+10=40 pounds/min. We have then that the two machines together can transfer 300 pounds in 300/40=7.5 minutes.