It is given in the question that initial amount of C 14 is 10 gms .
So after first half life,initial amount of C 14 becomes half , that is 5 grams .
After second half life, the amount left is the half of the amount that we got after first life, that is half of 5 gm which is 2.5 grams .
So after two half lives, the 10gm C 14 becomes 2.5 gm C 14 .
