Question

A solid of relative density 2.5 is found to weigh 10 g in water. find its weight in air?

Answer 1

relative density = density of object/density of water.
2.5= d.obj/10
d.obj= 25g.
weight in air = 25-10=15.
and you're done with a physics answer

Answer 2

we know that :
relative density of a object = density of object / density of water
given : relative density = 2.5
⇒ density of object = 2.5 * 10
∴ density of object = 25

⇒ weight in air = density of object - weight found in water 
                        = 25 - 10 
                        = 15
∴ the weight of object in the air = 15 g