Radioactive isoptope X has a half life of 4.6 Billion years. If at the beginning the amount of isotope X present was 10 grams, how long will it take

Radioactive isoptope X has a half life of 4.6 Billion years. If at the beginning the amount of isotope X present was 10 grams, how long will it take for isotope X to decai to .625 grams? 

Select one:

a. 18.4 billion years

b. 23 billion years

c. 9.2 Billion years

d. 4.6 billion year