If half life of an element is $$69.3 \mathrm{~h}$$, then how much of its percent will decay in 10th to 11th $$\mathrm{h}$$. Initial activity $$=50 ~\mu \mathrm{~Ci}$$

Assertion : Heavy water is used to slow neutron in nuclear reactor.

Reason : It does not react with slow neutron and mass of deuterium is comparable to the neutron.

Assertion : For an element generally $$N \geq Z$$ ( $$N=$$ number of neutrons, $$Z=$$ atomic number)

Reason : Neutrons always experience attractive nuclear force.

The half-life of a radioactive substance is $$20 \mathrm{~min}$$. The approximate time interval $$\left(t_2-t_1\right)$$ between the time $$t_2$$, when $$\frac{2}{3}$$ of it has decayed and time $$t_1$$ when $$\frac{1}{3}$$ of it had decayed is