Suppose that a radioactive decay chain consists of a parent isotope N1 with a half-life of 1 billion

Suppose that a radioactive dry-rot obligation consists of a inventor isotope N1 after a while a half-life of 1 billion years and three radioactive daughter isotopes N2, N3, and N4 after a while half-lives of 1 favorite years or hither. Write down an expression for provisionally activities of the filthy isotopes in the dry-rot obligation after 10 favorite years.