Maria throws two stones from the top of the edge of a building with a speed of 20m/s. She throws one straight down and the other straight up. The first one hits the street in a time t1 . How much later is it before the second stone hits?

Daughter element the new element produced along with a decay particle in a nuclear transmutation 2. half-life the substance that decays in a nuclear transmutation 3. parent element the change of one chemical element into another by nuclear decay or radioactive bombardment 4. transmutation the time required for the decay of one-half of the atoms in a sample of radioactive material