Suppose you find a rock that contains 10 micrograms of ...

[tsand2]

Suppose you find a rock that contains 10 micrograms of radioactive potassium-40, which has a half-life of 1.25 billion years.
 
  By measuring the amount of its decay product (argon-40 ) present in the rock, you conclude that there must have been 80 micrograms of potassium-40 when the rock solidified. How old is the rock?
  A) 3.75 billion years B) 2.5 billion years C) 5.0 billion years D) 1.25 billion years

Question 2

What is Cygnus X-1?
 
  A) the leading candidate for an observable black hole binary system
  B) a millisecond pulsar with three Earth-like planets around it
  C) the brightest star in the constellation Cygnus
  D) the first gamma-ray burster to be spotted in other wavelengths as well
  E) the strongest X-ray eclipsing binary system in the sky

[ricroger]

Answer to Question 1

A

Answer to Question 2

A

[tsand2]

Excellent

[matt95]

Great answer, keep it coming