Answer :

28 fish 

1.5 man gets 1.5 fish in 1.5 day
so => 3 men get 3 fish in 1.5 day
so => 6 men get 6 fish in 1.5 day
so => 6 men get 4 fish in 1 day
so => 6 men get 4*7= 28 fish in 7 day