Let us assume the number of boards that the carpenter started with = x
Length of each board that the carpenter cuts = 3/5
Then
x - 3x/5 = 4
(5x - 3x)/5 = 4
2x/5 = 4
2x = 4 * 5
2x = 20
x = 20/2
= 10
So from the above deduction, we can conclude that the carpenter started with 10 boards.I hope the procedure is not that complicated and it is easy for you to understand.