How do you want your little game to run?
I mean, you want to ask ten random math questions before the game is over. Do you want to keep track of how many the user gets correct and then report that score to the user after the ten questions?
You could put everything in a for/while loop and make it run 10 times (10 questions). And inside this loop keep track of how many times the user gets the math question correct with a variable, and then print out that score to the user outside your main game loop.
Also, if you want to print out a random math operator along with random operands (numbers), you can put your operators into an array of Strings and then print out a random index of that array.
Example:
//Declare and construct variables
Random randomnum = new Random();
int fnum, snum, answer;
int mathnumber = randomnum.nextInt(20);
int newnumber = randomnum.nextInt(20);
//Declare the operator
String[] operators = {"+", "-" , "/" , "*" };
int randomIndex = randomnum.nextInt(3);
String selectedOperator = operators.substring(i, i+1);
//Declare and construct variables of math problem
fnum = mathnumber;
snum = newnumber;
if(selectedOperator.equals("+")
answer = fnum + snum;
else if(selectedOperator.equals("-")
answer = fnum - snum;
else if(selectedOperator.equals("/")
answer = fnum / snum;
else if(selectedOperator.equals("*")
answer = fnum * snum;
//Output of math problem
System.out.print(fnum);
System.out.print(selectedOperator);
System.out.println(snum);