I think this is a least common multiple question
So they would all arrive on day 504, (which probably is a few years)
8 = 2 × 2 × 2
18 = 2 × 3 × 3
21 = 3 × 7
I might be right I might be wrong, IM not positively sure but yknow
SE NÃO ME ENGANO A RESPOSTA É A 10
The "CORRECT" answer is 746 with a remainder of 1
I hope this helps.
Have a "AWESOME" day!
:)