Question

a piece of iron weighs 44.5gf in air 39.5gf in water and 40.3gf in an oil find its relative density of iron

Answer 1

Relative density of a solid = weight of solid in air/ apparent loss of solid in air and in a liquid.
therfore the realtive density is 8.9.

Answer 2

Wa = weight of iron piece in air
Ww = weight of iron piece in water

normally relative density is calculated wrt  water.

relative density of iron piece =  Wa / (Wa - Ww)
         = weight in air / Loss of weight in water
         = 44.5 / (44.5 - 39.5) = 8.9
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if weight in oil is also known then you could calculate relative density of oil also.

relative density of oil = (Wa - Woil) / (Wa - Ww)