Aradioactive substance decays according to the formula a=a0e^kt
where a0 is the initial amount of substance (in grams) a is the amount of substance(in grams) after t years
k is a constant the half-life of the substance is 10 years. if we begin with 20g of the substance, how much will be left after 5 years?

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