standard_description: Determine the unknown whole number in an addition or subtraction equation relating to three whole numbers. For example, determine the unknown number that makes the equation true in each of the equations 8 + ? = 11, 5 = �� – 3, 6 + 6 = ��.
problem_activity: Potassium-40 has a half-life of
$${1.28\times 10^9}$$
years, which means that after that many years, half of a sample has decayed into a different isotope of carbon. If a sample began with
$${115}$$
grams of potassium-40
$${30,000,000}$$
years ago, how much is left today? How long will it be until there are only
$${50}$$
grams remaining?


aligns: no