Answer :
the fastest method would be to add 8+5 and then divide 390 by it. And then that would be your base unit which you multiply to 8 and 5 to get your answers
8+5=13
390/13
30
30x8=240 adults
30x5=150 children
8+5=13
390/13
30
30x8=240 adults
30x5=150 children