If all six pits are dug, (6-4)× 2 =4 pits will be dug more.
If all five pits are dug, three pits will be dug less (each person digs five tree pits, so there are three more).
Then we can find that the number of people digging the pit is (4+3)÷(6- 1)=7 (people).
The number of tree pits is 5×7+3=38.
Ask again if you don't understand. I wish you progress in your study and go up a storey still higher! (*^__^*)