A rock is suspended by a light string. When the rock is in air, the tension in the string is $52\mathrm{.}9$ N$.$ When the rock is totally immersed in water, the tension is $34\mathrm{.}6$ N$.$ When the rock is totally immersed in an unknown liquid, the tension is $20\mathrm{.}4$ N

$.$

Part A

What is the density of the unknown liquid?

Express your answer with the appropriate units.

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