If originally there were X atoms of radioactive isotope present and now there are only X/2 atoms, ...

If originally there were X atoms of radioactive isotope present and now there are only X/2 atoms, how many half-lives have elapsed?
 
  A) 3
  B) 1
  C) 4
  D) 2

Question 2

In grinding a steel knife blade (specific heat = 0.11 cal/g-C), the metal can get as hot as 400.C. If the blade has a mass of 80. g,
 
  what is the minimum amount of water needed at 20C if the water is not to rise above the boiling point when the hot blade is quenched in it?

Question 3

What is the area of a circle of radius 1.119 m?
 
  A) 3.934 m2
  B) 3.93 m2
  C) 3.9338 m2
  D) 3.93378 m2
  E) 3.937 m2

Question 4

A sphere of diameter 7.9 cm is completely submerged in water and seems to weigh 32.0 less in the water than out of it.
 
  (a) What is the density of the sphere?
  (b) If the sphere had been twice as dense, it would have weighed what  less in water than out?