Radioactive substances do not emit

Activity of radioactive element decreased to one third of original activity ${R_0}$ in $9$ years. After further $9$ years, its activity will be

Draw a graph showing the variation of decay rate with number of active nuclei. 

If one starts with one curie of radioactive substance ($T_{1/2}  = 12\,hrs$) the activity left after a period of $1$ week will be about

A radio-isotope has a half- life of $5$ years. The fraction of the atoms of this material that would decay in $15$ years will be