Two lighthouses flash their lights every 20 seconds and 30 seconds respectively. Given that they flash together at 8 p. m, when will they next flash together?

The first one will flash at every 30 sec and the second one at 20 sec. The second one will go like 1 - 4 - 8 and then to 12. By the time the second one will reach first at 6 and then on 12. So, after every 1 minute they will flash together.