As cities in the United States grow, they are finding that the ...

[SO00]

As cities in the United States grow, they are finding that the increased water demands are best addressed by addressing the water footprint, most often including
 
  A) building more reservoirs.
  B) drilling more wells.
  C) building desalination plants.
  D) conservation measures.

Question 2

Assume that a particular radioactive-isotope dating method has a half-life of 150 million years. Also assume that the parent isotope is commonly incorporated in igneous-rock minerals but that the daughter isotope is not.
 
  Measurements show that one crystal in an igneous rock contains 5 million atoms of the parent isotope and 75 million atoms of the daughter isotope. How old is this crystal?
  What will be an ideal response?

[braelync]

Answer to Question 1

D

Answer to Question 2

5 million parent isotope + 75 million of the daughter isotope = 80 million parent isotope as the clock starts
Parent Daughter Half-lives
80 million 0 0
40 million 40 million 1
20 million 60 million 2
10 million 70 million 3
5 million 75 million 4
4 half-lives have gone by. If each half-life takes 150 million years, then the age of the crystal is 600 million years old (150 million years/half-life x 4 half-lives = 600 million years old).

[SO00]

Excellent

[bulacsom]

Gracias!