The Mean Life (Time Constant) of Radon-222 Is $5.526 \mathrm {~d}$

The mean life (time constant) of radon-222 is $5.526 \mathrm {~d}$ . If a sample of radon initially contains $6.00 \times$ $10 ^ { 8 }$ radon atoms, how many of them are left after $10.0 \mathrm {~d}$ ?

A) $8.34 \times 10 ^ { 8 }$
B) $7.29 \times 10 ^ { 8 }$
C) $9.82 \times 10 ^ { 7 }$
D) $7.67 \times 10 ^ { 7 }$
E) $8.56 \times 10 ^ { 8 }$