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 :
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 : This exercise asks to write a chain of
: Swap the terms and express the second as a difference ( Step-by-step : This exercise asks to write a chain of