By adding 104+34 and then 228+877, Then adding both of the results and finally subtracting the answer by one of the factors.
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Explanation:
Which will be...
104+34=138
228+877=1,105
Then add them...
138+1,105=1,243
Finally, subtract your answer by one of the factors...
1,243-1,105
Which will result as 138. Which is also one of the factors you added earlier, that's basically what your looking fro when checking and subtraction and/or addition problem.
