Sandeep + Sister = 54.75 kg
Sandeep + Mother = 105 kg

Both “Sandeep”s are the same, so it has to be the case that the Mother is 105 - 54.75 = 50.25 kg heavier than the Sister.

Since the Mother is 4 times as heavy as the Sister, the mother’s mass is 4 units and the sister’s mass is 1 unit. The difference is 3 units.

3 units —> 50.25 kg
1 unit —> 50.25 divided by 3 = 16.75 kg

This means that the Sister has a mass is 16.75 kg and the Mother has a mass of 16.75 x 4 = 67 kg.

Since Sandeep and Sister has a total mass of 54.75 kg, Sandeep’s mass = 54.75 kg - 16.75 kg = 38 kg.

You can check also from the other condition that Sandeep and Mother has a total mass of 105 kg, so sandeep’s mass is 105 - 67 = 38 kg as before.