Double * double is giving int as output in cpp

No, it's not! :laugh:
It's just that cout defaults to an "unformatted double" which removes trailing zeros after the decimal point.
If you change your code to this:

while(t--){
    double n=0.2;
    cout<<n*3.00<<"\n";
}

It will print "0.6" t times.
You can also use fixed to "force" decimal places:

cout<< fixed <<n*3.00<<"\n";

Which will give you "6.000000"
Or you could combine it with setting the precision to give you two decimal digits:

int main()
{
    cout<<"Hello World";
    int t;
    cin>>t;
    cout.precision(2);
    while(t--){
        double n=2.00;
        cout<< fixed <<n*3.00<<"\n";
    }
    return 0;
}