   SEARCH HOME Math Central Quandaries & Queries  Question from kevon, a parent: if x = 7 is used in the expression 2x + 5 what is the output Hi Kevon,

Two things to notice in this expression. First of all $2x$ means 2 times x so the expression can be seen as $2 \times x + 5.$ Secondly the letter $x$ is used to represent an unknown number. Let me illustrate with a different question.

If $x = 5$ is used in the expression $7x - 8$ what is the output?

Using the first of my two observations above this expression can be written $7 \times x - 8.$ Again the letter $x$ represents an unknown number. The question then tells you that $x = 5$ so $x$ is no longer unknown. Thus the expression is $7 \times 5 - 8.$ Now it's easy, $7 \times 5 = 35$ so the output is $7 \times 5 - 8 = 35 - 8 = 27.$

If the question had been

If $x = 3$ is used in the expression $7x - 8$ what is the output?

then the output would be different. It would be $7 \times 3 - 8 = 21 - 8 = 13.$

Now you try the question you sent,
Penny     Math Central is supported by the University of Regina and The Pacific Institute for the Mathematical Sciences.