Arock has 12.5 percent of its original amount of potassium-40 remaining in it; potassium-40 has a half-life of 1.25 billion years. how long ago was the rock formed?

a. 1.25 billion years ago

b. 2.5 billion years ago

c. 3.75 billion years ago

d. 5 billion years ago

If the rock has 12.5 % of its original amount of potassium-40 remaining in it, this has a half-life of 1.25 billion years, the rock was formed approximately 3.75 billion years ago. The correct answer is C. 

Answer is: the equilibrium  shifts toward the reactants, to the left side of chemical reaction.it is because lead(ii) nitrate dissociates in water: pb(no₃)₂ → pb²⁺(aq) + 2no₃⁻(aq).according to lechatelier's principle, the position of equilibrium moves to counteract thechange (more lead cations, common ion), the position of equilibrium will move so that the concentration ofproducts decrease and concentration of reactants of chemical reaction increase.read more on -