The correct expression for relative density when the weight of a body in air is W₁, and in water is W₂ is given by :

1. R.D. = $\dfrac{\text{W}$2}{\text{W}1 - \text{W}_2}

2. R.D. = $\dfrac{\text{W}$1}{\text{W}1 + \text{W}_2}

3. R.D. = $\dfrac{\text{W}$1 + \text{W}2}{\text{W}_1}

4. R.D. = $\dfrac{\text{W}$1}{\text{W}1 - \text{W}_2}

R.D. = $\dfrac{\text{W}$1}{\text{W}1 - \text{W}_2}

Reason — R.D. = $\dfrac{\text{Weight of body}}{\text{Weight of water displaced by the same body}}$

= $\dfrac{\text{Weight of body}}{\text{Loss of weight of body in water (or upthrust)}}$

$\dfrac{\text{Weight of body in air (W)}$1}{\text{Weight of body in air (W)}1 - \text{Weight of body in water (W)}_2}

Thus,

R.D. = $\dfrac{\text{W}$1}{\text{W}1 - \text{W}_2}