Explanation:
we need the least or smallest common multiple for both numbers, 5 and 6.
this is very simple for the small numbers here, but in general we look at the prime factors of both numbers :
6 has the prime factors 2 and 3.
5 has only 5.
so, the LCM is 2×3×5 = 30.
they will meet again after 30 days.
FYI -
the LCM is for larger numbers a and b not automatically a×b. if both numbers share prime factors, the LCM can be smaller than a×b, as the LCM is using every overlapping prime factor only once.