Using the following data to complete the problems below:

Dating System. Age of half life
238 U->206 Pb 4.5 billion years
235 U -> 207Pb 713 million years
40k-> 40Ar 1.3 billion years
14C-> 14N. 5,730 years
A rock originally had 4800 atoms of 40K and no 40Ar. It now has 400 atoms of 40K.
Assuming that the only chemical process is radioactive decay of potassium to
argon, what is the age of the rock?
a) 1.576 billion years
b) 1.375 billion years
c) 2.643 billion years
d) 4.660 billion years
e) 5.303 billion years