2 *140+3 *130+4 *120 =1150 trees
2) 1520/2=760
2* 140+4* 120=760
Send level 4 and level 6 to 1 hour to complete the task.
3)1167/3+1= 399 trees.
399*2=789 trees
Because 130*3=390.
390*2=780
It takes more than two hours to send the fifth grade to complete the task.