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You haven't given the first liquid. Since relative density is required, i am assuming it to be water.
Since in both cases the test tube floats upto mark X, the volume displaced in case of brine and water are same. 
Let density of water = w
density of brine = b
Relative density of brine = b/w

So Volume displaced in water = Volume displaced in brine

⇒  \frac{25}{w*g} = \frac{30}{b*g}

 \frac{b}{w}= \frac{30g}{25g}  = \frac{30}{25}

 \frac{b}{w}= \frac{30}{25}=1.2

Thus relative density of brine is 1.2
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