upthrust = weight of the volume of water displaced by stone

= m\times g

= mass of displaced water \times g

the volume of stone equal to the volume of water displaced. thus the volume of displaced water is 200

then the mass of displaced water is = density of liquid x volume

m = 1.20\times 200 = 240 g

upthrust = 240/ 1000\times 10

= 2.4 N

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