## Usage of Unknown

 1.8 Usage of Unknown

 Unknown in Subtraction
 SITUATION: Ali read 9 pages of a story book on Monday. On Tuesday, he read a few pages more. The total of pages he has read is 20 pages.  Number of pages Ali read on Tuesday is unknown. SOLUTION: Monday: 9 pages Tuesday: $$x$$ pages (unknown) Total pages: 20 pages If we change to mathematic equation, we will get $$9 + x = 20$$. To find $$x$$, we will subtract 9 from both parts of the equation.  $$9 \color{red}- \color{red}9 +x = 20 \color{red}- \color{red}9$$ $$x = 11$$ Therefore, $$x$$ is 11.

Unknown in Subtraction

 SITUATION: Ali collects stamps. He gives 12 stamps to his brother. Ali now has 38 stamps. Number of stamps Ali collects is unknown. SOLUTION: Stamps that Ali collect : $$y$$ pieces Stamps that Ali give to his brother: 12 pieces Remainder of stamps that Ali have: 38 pieces If we change to mathematic equation, we will get $$y - 12 = 38$$. To get $$y$$, we will add 12 to both sides of the equation. $$y -12 \color{red}+\color{red}1\color{red}2 = 38 \color{red}+\color{red}1\color{red}2$$ $$y = 50$$ Therefore, $$y$$ is 50.