Translating phrases and sentences into algebraic form

 Example 1: Lets practice translating phrases into algebraic form. Consider "seven more than three times a number". What do the words "seven more than" mean?

Seven more than means to add 7.

What will the "7" be added to?

It will be added to whatever follows the words "more than". Here it is the rest of the phrase: "three times a number".

Using "x" to represent "a number" finish translating the phrase "seven more than three times a number".

Either (7 + 3x) or (3x + 7) is correct because addition is commutative.

Let's change one word in the original phrase. Let's change "more" to "less":

" seven less than three times a number"

What do the words "seven less than" mean?

"seven less than" means to subtract 7.

What do we subtract 7 from?

As before, it is the description following "than".

Using "x" to represent "a number" finish translating the phrase "seven less than three times a number". 

3x - 7 is the translation of "seven less than three times a number".

Notice how the order of the algebra is different from the order of the words. We say the words with the "seven less than" first, but have to write the "- 7" as the second term. Order matters in subtraction.

Example 2: Now consider a phrase mentioning two numbers.

"There are two numbers, the larger of which is four more than three times the smaller."

We want to represent the numbers in terms of one variable, "x".

Which number should "x" represent?

Since the larger number is described in terms of the smaller number, it is clearer to use "x" as the smaller number.

Using "x", translate "three times the smaller".

"three times the smaller" involves multiplication of 3 and x.

Now consider the "four more than". What mathematical operation does this involve?

"more than" involves addition.

Now finish translating "There are two numbers, the larger of which is four more than three times the smaller." by describing the larger number in terms of "x".

Either 4 + 3x or 3x + 4 describes the larger number.

Now let's try a phrase which describes three numbers:

"There are three numbers. The middle number is 5 times the smallest, and the largest number is 8 more than the middle number."

We want to describe the three numbers in terms of a single variable, "x". What is a good choice for "x"?

Since the smallest number is used to describe the middle one, and the middle one is used to describe the largest one, a good choice for "x" is the smallest number.

Using "x" as the smallest number, describe the middle number as "5 times the smallest".

The word "times" means multiplication, so the middle number is 5x.

Now describe the largest number as "8 more than the middle number".

Since the middle number is 5x, we need to add 8 to it to get 8 + 5x.

In summary, the three numbers are x, 5x, and 8 + 5x.