In ten years, Mark will be  years old, so Mark is  years old now, and Brian is one-third of this, or  years old. 

Let  be the number of years in which Mark will be twice Brian's age. Then Brian will be , and Mark will be . Since Mark will be twice Brian's age, we can set up and solve the equation:

Mark will be twice Brian's age in  years.

Since Gary will be 37 in five years, he is  years old now. He is twice as old as Cathy, so she is  years old, and in five years, she will be  years old.

This problem involves two seperate multiplication problems. Erin will make $450 at the reunion but supplies cost her $300 to make the shirts. So her profit is $150.

Let  represent the unknown quantity.

The first expression:

"The product of a number and three" is three times this number, or 

"Ten added to the product" is

The second expression:

"Twice the number" is two times the number, or

.

The desired equation is therefore

.

"The difference of a number and nine" is the result of a subtraction of the two, so we write this as

"Five-sevenths of" this difference is the product of  and this, or

This is equal to forty, so write the equation as

Let  represent the unknown number.

"The sum of a number and ten" is the expression . "Twice" this sum is two times this expression, or

.

"The difference of the number and one half" is a subtraction of the two, or

Set these equal, and the desired equation is

In order to solve this problem, we need to recall the formula for perimeter of a rectangle:

We can substitute in our known values and solve for our unknown variable (i.e. length):

We want to isolate the  to one side of the equation. In order to do this, we will first subtract  from both sides of the equation. 

Next, we can divide each side by 

The length of the rectangle is 

In order to solve this problem, we need to recall the formula for perimeter of a rectangle:

We can substitute in our known values and solve for our unknown variable (i.e. length):

We want to isolate the  to one side of the equation. In order to do this, we will first subtract  from both sides of the equation. 

Next, we can divide each side by 

The length of the rectangle is 

In order to solve this problem, we need to recall the formula for perimeter of a rectangle:

We can substitute in our known values and solve for our unknown variable (i.e. length):

We want to isolate the  to one side of the equation. In order to do this, we will first subtract  from both sides of the equation. 

Next, we can divide each side by 

The length of the rectangle is 

In order to solve this problem, we need to recall the formula for perimeter of a rectangle:

We can substitute in our known values and solve for our unknown variable (i.e. length):

We want to isolate the  to one side of the equation. In order to do this, we will first subtract  from both sides of the equation. 

Next, we can divide each side by 

The length of the rectangle is