The answer is 73. 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.
45
234 45 279 109 388 28 416 37 453 227 680
-73 + 28 = -45 |-73| - |28| = 73 - 28 = 45 (Since -73 has the largest absolute value keep its sign)
7x+17 = 45 7x = 45-17 7x = 28 x = 4
The country code for Denmark is +45, so the second one is possible. Country codes +28 and +950 are invalid.
6/7 + 3/4 = 45/28 or 117/28
-45 plus 512 plus 45 plus 56 = 568
The sum of 2/5 and 2/9 is 28/45.
45 - 28 = 17.
28
45 x 28 = 1260
12 + 23 + 45 + 67 + 16 + 18 + 45 = 226