To answer this question we will use trial and error
Naomi caught 2 times less fish
So if Jack caught 6
Naomi caught 3
Total 9
So if Jack caught 8
Naomi caught 4
Total 12
So if Jack caught 10
Naomi caught 5
Total 15
So if Jack caught 12
Naomi caught 6
Total 18. <—————
Therefore Jack caught 12 fish