Intermath | Workshop Support


Double Trouble

Problem Statement
What number can you add to 129 and subtract from 129 such that the sum of the number and 129 is twice the difference between 129 and the number?


Plans to Solve/Investigate the Problem

I plan to use an equation to solve the problem.


Investigation/Exploration of the Problem

I let x = the unknown number.  My equation was x + 129 = 2(x – 129).  My solution was 387.  Lisa said the solution was 43. Her solution did not work in my equation. Upon comparing equations we disagreed with the difference of 129 and the number. Discussion followed to determine the meaning of the difference of 129 and the number. Did it mean x – 129 or 129 – x. The solution 387 did not meet the criteria stated in the problem that number must be subtracted from 129. Thus the only solution was 43.


Dottie Mitcham

Link(s) to resources, references, lesson plans, and/or other materials
Link 1
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