Answer: 40 times
Explanation:
Given: Total planks = 20
By using integers, Steps forward is represented by +5, Steps backward s represented by -4.
After first set of two steps ( one forward another backward), 5-4 =1 step forward is captured.
For 20 planks , we require 20 such sets, i.e. he will do this 2x 20 times i.e. 40 times.
Hence, he will do this 40 times to reach the other side.