A "drug dissipates at a continuous rate" 12% per hour. how long until a quarter of the drug is left.

It would take 6 hours and 15 minutes.

Add 12 to itself until you reach near 75%. 6 x 12 = 72

This leaves 3 % still to go. 3% is one fourth of 12, so it would be 6 and 1/4 hours which is 6hrs and 15 min.