MATH PUZZLE: Can you replace...

After years of working for others and being passed over for promotions, Blonde John and his wife, Blonde Mary, decided to go into business together. After examining the classifieds, they bought a small candy stand, paying thirty cents for each box of candy and then selling it for thirty cents.
At the end of the day they were astonished to find that they had sold every box of candy – yet had exactly as much money as when they started.
“You see?” John snarled at his wife. “I told you we should have bought a larger stand!”