A electron of hydrogen atom on an excited state is having energy $$\mathrm{E}_{\mathrm{n}}=-0.85 \mathrm{~eV}$$. The maximum number of allowed transitions to lower energy level is _________.

A capacitor of capacitance $$\mathrm{C}$$ and potential $$\mathrm{V}$$ has energy $$\mathrm{E}$$. It is connected to another capacitor of capacitance $$2 \mathrm{C}$$ and potential $$2 \mathrm{~V}$$. Then the loss of energy is $$\frac{x}{3} \mathrm{E}$$, where $$x$$ is _______.

The distance between object and its two times magnified real image as produced by a convex lens is $$45 \mathrm{~cm}$$. The focal length of the lens used is _______ cm.

In a closed organ pipe, the frequency of fundamental note is $$30 \mathrm{~Hz}$$. A certain amount of water is now poured in the organ pipe so that the fundamental frequency is increased to $$110 \mathrm{~Hz}$$. If the organ pipe has a cross-sectional area of $$2 \mathrm{~cm}^2$$, the amount of water poured in the organ tube is __________ g. (Take speed of sound in air is $$330 \mathrm{~m} / \mathrm{s}$$)