How to check addition problems

I show two basic techniques for checking your own work when you're adding many numbers in columns (standard addition algorithm). The first one is, add the individual digits in a different order. The second is, first round each number and add those to get an estimate. Then add your original numbers and compare your result to the estimate. If there is a big difference or discrepancy, there might be an error.
Online practice for vertical (column) addition:
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