Writing equation in one variable

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An equation is a mathematical sentence that uses an equal sign, =, to show that two expressions are equivalent.

To write a word sentence in the equation form, look for keywords or phrases like “is,” “the same as,” or “equals” to find out where to put the equal sign.

Example 1:

Tom has $17 in his piggy bank. How much money does he need to buy a game that costs 68?



Let x be the amount of money Tom needs. Therefore, we can represent this problem with the following equation:

17 + x = 68


Subtract 17 from both sides of the equation to find the value of x

68 - 17 = x

Answer: x = 51; Tom needs $51 to buy the game.

Example 2:

Write the word sentence as an equation.

The sum of a number n and 7 is 15.


As “the sum of” means addition, rewrite “the sum of a number n and 7” as n + 7.

n + 7 = 15


Solve for x

15 - 7 = 8

Answer: n = 8