A carpenter is cutting a wooden beam to make the rungs of a special ladder. The length of the first rung is 150 cm, and each successive rung is 2 cm shorter than the previous one.
(i) Determine the common difference of the sequence formed by the rung lengths.
(ii) Write a formula for the length of the nth rung.
(iii) Find the length of the 40th rung.
(iv) After how many rungs will the length become less than 50 cm? Show all steps clearly.