The velocity of an electron in a certain Bohr’s orbit of H-atom bears the ratio $1:550$ to the velocity of light. The quantum number $n$ of the orbit is

Given that

Velocity of electron $=\frac{1}{550}\times$ velocity of light

But we know velocity of electron in Bohr orbit is $V_n=\frac{2.19\times {10}^8\times Z}{n}cm/\sec$

$\therefore \frac{2.19\times {10}^8\times 1}{n}=\frac{1}{550}\times 3\times {10}^{10}$

$\therefore n=\frac{2.19\times {10}^8\times 550}{3\times {10}^{10}}=4$

Hence answer is 4.