Question

A radioactive substance decays according to the formula A=A0e^kt

Answer 1

Without needing fancy formulas, we can conclude that after 10 years, half of the substance will be left.  So, we can make our own formula:
amount remaining = 1 / 2^(years/10)
So, after 5 years
amount remaining = 1 / 2^(5/10)
amount remaining = 1 / 2^(.5)
amount remaining = 1 / 1.414213562
amount remaining = 0.70711 * 20 grams
amount remaining = 14.1421356237 grams