I believe -3(2n)2 is equal to -6n(2)
1 bag of 64
2 bags of 32
4 bags of 16
Or 8 bags of 8
One minute later at 8:01pm. One lighthouse will flash 3 times (at 20, 40 and 60 seconds)...the other will flash twice (at 30 and 60 seconds). So they will both flash 60 seconds later, at 8:01