Alocal department store had a saturday special. it offered an additional 25% discount on shirts that were already marked down 50%. how much did megan pay for a shirt that originally sold for $40?

There were first 50% discount on a $40 shirt, 
Compute 50% of $40 like this:
0.50 * 40 = $20.
The new price of the shirt is then $20.
Another 25% discount of $20 equate the following:
0.25 * 20 = $5.
Substract $5 from $20 in order to get the new price of the shirt:
$20 - $5 = $15

$15