//------------------------------//
// The Answer 5 
// Story: Life is a Test: A Series of Pony Logic Puzzles 
// by Brony_of_Brody 
//------------------------------//

Add another AS pill to the pile, then grind the pills into a powder.

All Derpy has to do after that is take half of the mixture and save the other half for the next time she gets poisoned.

OR

Add another AS pill to the pile, then cut all the pills in half and divide them into 2 piles. (Make sure you have one half of the AS pill you added in each pile)

Derpy can then take one of the pill piles and save the other half for next time. You might think this is problematic since the pills are indistinguishable, but now you've added another AS pill it doesn't make a difference.  While she's talking the first half she will always have the equivalent of one AR and one AS pill.

"NEXT TIME!?"