A prescription calls for 25mg of a medication t.i.d. for 7 days. The stock solution is composed of a 100mg/3mL. How many Liters of the stock solution is needed for the order?

The abbreviation t.i.d. means 3 times a day. So 7 days x 3 times per day x 25 mg each = 525 mg total needed.
100/3 = 525/x
Cross multiply to solve for x:
100x = 1575
x = 15.75 mL.
The answer asks for liters so divide by 1000:
15.75/1000 = 0.01575 L