A radioactive nucleus A with a half life T, decays into a nucleus B. At t = 0, there is no nucleus B. At sometime t, the ratio of the number of B to that of A is 0.3. Then, t is given by :

The following observations were taken for determining surface tension T of water by capillary method : Diameter of capillary, D = 1.25 × 10 ⁻² m rise of water, h = 1.45 × 10 ⁻² m Using g = 9.80 m/s² and the simplified relation T = (rhg /2)