ul: [[10,99],[2,12]],
            div: [[2,12],[10,99]],
          },
          hard: {
            add: [[100,999],[100,999]],
            sub: [[100,999],[100,999]],
            mul: [[10,99],[10,19]],
            div: [[2,19],[10,99]],
          }
        };

        if (op === 'add') {
          a = pick(ranges[diff].add[0]);
          b = pick(ranges[diff].add[1]);
          ans = a + b;
          text = `${a} ${symbols.add} ${b} =`;
        } else if (op === 'sub') {
          a = pick(ranges[diff].su