THANK YOU MAN This helped me a lot. I'm using String instead of StringBuilder or StringBuffer. Now I changed it to StringBuffer and my code ran faster than before :) This can still be reduced to:

public static void main(String[] args) {
    Scanner in = new Scanner(System.in);
    int t = in.nextInt();
    for(int a0 = 0; a0 < t; a0++){
        {
            int n = in.nextInt();
            StringBuilder strb=new StringBuilder();
            for(int i=n;i>=0;i--)

            {   
                if(i%3==0 && (n-i)%5==0)
                {
                    int j=0;
                    for(j=0;j<i;j++)
                        strb.append("5");
                    for(int k=j;k<n;k++)
                        strb.append("3");
                    break;

                }
            }
            if(strb.length()==0)
                System.out.println("-1");
            else
                System.out.println(strb);
        }
    }
}