The Activity of a Sample Reduces From $7.55 \times 10 ^ { 9 } \mathrm {~Bq}$

Short Answer

The activity of a sample reduces from $7.55 \times 10 ^ { 9 } \mathrm {~Bq}$ to $5.11 \times 10 ^ { 9 } \mathrm {~Bq}$ in $5.50$ months.
(a) What is its half life?
(b) How many months from the start must one wait for the activity to decrease to $10 \%$ of its original value?