Learn about Expressions with Addition and Subtraction Addition and subtraction of expressions: In this problem, it states that the adult members of an environmental club plant a total of five trees. Each of the young people plants one tree. How many trees are planted if there are eight young people? In this problem, we need to write and expression for the number of trees planted. We know that the number of trees planted by the adult members is five. We also know that each young person planted one tree. We’re going to let the number of young people in the environmental club be Y. So, we’re going to use the variable for Y to take the place of the number of young people in the environmental club. The number of trees planted by the young will be Y, so if we let the number of young people in the environmental club be Y, then the number of trees planted by the young will be Y. So our equation will be the total number of trees planted is going to equal the number of trees planted by the adult members plus the number of trees planted by the young people. So therefore, our total here, the total number of trees planted will equal our number of trees by the adults is five plus and the trees planted by the young is Y. So our expression is Y+5. Now the expression gives us the total number of trees planted. Note that the total number of trees planted will vary with the number of young people. As the number of young people change, the number of trees planted will change. We know that the number of young people is eight. So, we know that there are eight young people. Now, we can find the number of trees planted by evaluating 5+Y and putting Y=8. So therefore, the number of trees planted that the total number of trees planted will equal 5+8 because we’re putting 8 in for Y. It would be 5+8 and 5+8 is 13, so 13 trees are planted if there are young people. Now, we can also use a model to solve this. Let’s take a look at the model. So we know that the adult members planted five trees. So here, we have the adult members planted five trees. We’re going to add a counter for each tree planted by each of the young members. So, we’re going to add counters to each tree planted. So we have our five adults. So now each tree planted for each young adult. You notice for five young people we have five trees, for six young people will be adults plus the six young people, adults plus the seven young people and adults plus the eight young people. The number of trees is equal to the number of counters. So the numbers of counters used when there are eight young people are 13. So we have 13 counters which means that there are 13 trees. Things to remember and to keep in mind; is that algebraic expressions can be used to describe situations. An unknown quantity can be represented using a variable in the expression. The value of the expression changes with the change in the value of the variable.