If $87.5 \%$ of atoms of a radioactive element decay in 6 days, then the fraction of atoms of the element that decay in 8 days is

If the ratio of the mass numbers of two nuclei is $27: 125$, then the ratio of their surface areas is

The range of weak nuclear force is of the order of

The potential energy of an electron in an orbit of hydrogen atom is -6.8 eV . The de-Broglie wavelength of the electron in this orbit is

( $r_o$ is Bohr radius)