諗左條題目好耐,有d明...但係又好混亂,麻煩各位幫幫手

Three ships leave New York for France on the same day.
The round trip takes the first ship twelve days, the second ship sixteen days,
and the third ship twenty days. How many days will elapse before all three ships
again leave New York on the same day? Assume continuous round-trip activities for
all three ships.

引用:

識唔識Least common multiple (LCM)

唔識,請師兄指教