It's VERY important you double check and make sure how long these are supposed to last. Since you mention the 15 days...then I am assuming the Dr. or someone expressed to you that this was how long they are to last.
As mentioned...it's not about the prescription directions on the bottle as those are the "maximum" amount taken. Most Drs. expect you to take a medication like these as needed for the really bad times...
If you do not check first before you take them all in 5 days...you may find yourself out of luck and with a Dr. who is upset that you didn't follow their instructions...
So....you can NEVER go wrong by double checking on things like this.
Your math is correct. Call your doctor and find out what's up. some doctors beleive a patient should not exceed a certain amount of pills in a given time period regardless of how they say to take the pills. you might have one of those doctors.
WHats this prescription for? that's a pretty crappy prescription i'd be pissed