Jen cell phone cost her $21 per month plus $3 for every 1 gb of data downloaded what is the limit to the number of gbs she can download to stay with her monthly budget of $30

She can use 3 GBs of data every month.
$3 * 3GB = $9
$9 + $21 = $30

Hope this helps.

We first need to write an inequality to solve this. Already she needs to pay 21 dollars per month which makes it the initial fee or in this case the y-intercept. We don't know how much GB of data she is going to download so that will be x. The inequality will look like this:

3x + 21 ≤ 30

Now we just solve for x. 
3x + 21 ≤ 30
      -21     -21
3x ≤ 9
3      3
x ≤ 3

She can only get 3GB or less of data to stay within the limit of $30 each month.