Question

The half life of a certain substance is about 4 hours. The graph shows the decay of a 50 gram sample of the substance that is measured every hour for 9 hours.

A graph shows time (hours) labeled 1 to 10 on the horizontal axis and quantity (grams) on the vertical axis. A line decreases from 0 to 9.

Which function can be used to determine the approximate number of grams of the sample remaining after t hours?

y = 25(0.15)t
y = 25(0.85)t
y = 50(0.15)t
y = 50(0.85)t

Answer 1

Use the exponential decay formula with half-life 4: y = 50*(1/2)^(t/4). Since (1/2)^(1/4) ≈ 0.8409 ≈ 0.85, the best choice is

y = 50(0.85)^t.