At room temperature, gaseous hydrogen is bombarded with a beam of electrons of 13.6 eV energy. The series to which the emitted spectral line belongs to

The half-life of a radioactive substance is 12 min . The time gap between $28 \%$ decay and $82 \%$ decay of the radioactive substance is

An element consists of a mixture of three isotopes $A, B$ and $C$ of masses $m_1, m_2$ and $m_3$, respectively. If the relative abundances of the three isotopes $A, B$ and $C$ are in the ratio $2: 3: 5$, the average mass of the element is