Answer:
We know that per week, she works 18 hours.
For each of these hours, she earns $13.
Then in one week, she earns a total of:
18*$13 = $234
This is her gross pay (the pay before the taxes apply)
Now we know that her employer takes:
15% for federal income tax withholding.
6.2% for Social Security
1.45% for Medicare.
If we add all of these, we get:
15% + 6.2% + 1.45% = 22.65%
This means that 22.65% of $234, is taken by her employer to pay taxes and such.
Then the amount left is:
M = $234 - ( 22.65%/100%)*$234 = $234 - 0.2265*$234 = $180.999
And we can round the last digit to $181 (because we can not have a 0.009 of a cent)
This means that her weekly net pay is $181
And she works for 12 weeks, this means that in total, she will earn:
12*$181 = $2,172