The answer is 1891. Simply add the numbers together arithmetically as you would for the problem two plus two. To check your work, a calculator is the best option.
892 + 564 = 1456
892
999
1998
999
999
999 + 9999999999 + 9999999999 + 8888888 = 20,008,889,885
999 999
999 + 36 = 1035
999 + 781 = 1780
999 + 99 = 1098
999 + 12,345 is 13,344.