Radium-226 is a common isotope on earth, but it has a half-life of about 1620 years. given that earth is some 5 billion years old, why is there any radium at all?

Radium-226 is a daughter isotope and the result of the uranium radioactive decay.

At each of their respective half-life cycles, Radium-226 and Astatine-218 are converted back and forth by transmutation.

i'm assuming that the correct answer is indeed the first one/ "a"..

for step 7 table c on the lab "newtons law of action"   the average time it took the car to travel .25 meters   was 1.92 seconds.

the average time it took the car to travel 0.50 meters was 2.61 seconds

hope this : )