2 buses leave at 5:30am, 1 comes every 18 minutes and one comes every 15 minutes when will they both

2 buses leave at 5:30am, 1 comes every 18 minutes and one comes every 15 minutes when will they both come at the same time again

We want the Least Common Multiple (LCM) of 15 and 18. LCM(15, 18)

Enter this into the search engine, and we get:
90 minutes