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: Swap the terms and express the second as a difference ( Step-by-step :

This exercise asks to write a chain of numerical equalities using specific mental calculation techniques. 1. Addition of a Two-Digit Number : Technique : Break the second number into tens and units ( Step-by-step : : Swap the terms and express the second

138+106=106+138=106+(140−2)=106+140−2=246−2=244138 plus 106 equals 106 plus 138 equals 106 plus open paren 140 minus 2 close paren equals 106 plus 140 minus 2 equals 246 minus 2 equals 244 3. Subtraction by Rounding Up : Technique : Round and add the difference back ( Step-by-step : Subtraction by Rounding Up : Technique : Round

87−49=87−(50−1)=87−50+1=37+1=3887 minus 49 equals 87 minus open paren 50 minus 1 close paren equals 87 minus 50 plus 1 equals 37 plus 1 equals 38 4. Direct Subtraction by Parts : Technique : Subtract the tens first, then the units ( Step-by-step : then the units ( Step-by-step :