The answer is 459. 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.
456 plus what equals 1242.\
456 plus 100 is equal to 556
123 + 456 + 7890 = 8469
898 + 456 = 1354
777568 + 456 = 778024
789 + 456 = 1245
66 + 456 = 522
4002 + 456 = 4458
456 + 312 = 768
456 + 789 + 123 + 123456789 = 123458157
456 + 656 = 1,112
123 + 456 = 579