Length of hydrometer stem = 10cm
volume of hydrometer stem = 10cm³
So base area (A)= 10cm³ / 10cm = 1cm²

When it is PUT in pure water, It sinks to 5cm mark
mass of hydrometer = 200g
since it is floating, mass of water DISPLACED = mass of hydrometer = 200g
assuming density of water = 1g/CC

volume of hydrometer(upto 5cm mark) = mass of water displaced/density of water                                                               =200/1
                                                           = 200 cc

now if it is put in a liquid where
(i) it sinks to the top of the stem,
the mark till which it sinks is 10cm.

volume of liquid displaced = volume of hydrometer(upto 5cm mark) + volume of hydrometer from 5cm mark to 10 cm mark
                                       = 200 cc + (10-5)cm×1cm²
                                       = 205cc
     [(10-5) is the height and 1cm² is the base area]
                                       
density of liquid = mass of hydrometer / volume of liquid displaced
                        = 200/205
                        = 0.9756 g/cc

(ii) it sinks to the bottom of the stem,
the mark till which it sinks is 0cm.

volume of liquid displaced = volume of hydrometer(upto 5cm mark) - volume of hydrometer from 0cm mark to 5 cm mark
                                       = 200 cc - (5-0)cm×1cm²
                                       = 195cc
     [(5-0) is the height and 1cm² is the base area]
                                       
density of liquid = mass of hydrometer / volume of liquid displaced
                        = 200/195
                        = 1.0256 g/cc