The puzzle asks you to complete 432 + 1?8 = 600. Only the middle digit of the second number is missing, so there is no need to test every possible combination.
Start with the total and subtract the number you already know: 600 minus 432 equals 168. The mystery number is therefore 168, which puts a 6 in the space marked by the question mark.
You can check it by adding the columns. Two plus eight makes ten, so write zero and carry one. Three plus six plus that carried one makes ten again. Carrying into the hundreds leaves four plus one plus one, which gives six. The result is exactly 600, confirming that the missing digit is 6.