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Here is the complete solution for problem from the 7th-grade Algebra textbook by Dorofeev, Suvorova, and Bunimovich.

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 : Here is the complete solution for problem from

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 : Addition of a Two-Digit Number : Technique :

256+54=256+(50+4)=256+50+4=306+4=310256 plus 54 equals 256 plus open paren 50 plus 4 close paren equals 256 plus 50 plus 4 equals 306 plus 4 equals 310 2. Commutative Property and Subtraction Method : Subtraction by Rounding Up : Technique : Round

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 :

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

94−61=94−(60+1)=94−60−1=34−1=3394 minus 61 equals 94 minus open paren 60 plus 1 close paren equals 94 minus 60 minus 1 equals 34 minus 1 equals 33 The complete results for the four expressions are: