What is the half life of a radioactive isotope that decreased to one-fourth its original amount in 100 year

Which one of the following research methods can show cause and effect

Individuals who live below the poverty line get seriously ill more often than those who do not what could be the hidden variable in this situation?

An air-filled capacitor is formed from two long conducting cylindrical shells that are coaxial and have radii of 26 mm and 103 mm. the electric potential of the inner conductor with respect to the outer conductor is - 700 v (k = 1/4πε 0 = 8.99 × 109 n · m2/c2) the energy stored in a 1.0-m length of this capacitor is closest to