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If the mass of the container is u kg when empty, weight of the water in full is 20.51 – u .

When water is half,

 ( 20.51 – u )/2  = 12.14          multiply both sides by 2

20.51 – u = 24.28

u = 3.77

So mass of the container is 3.77 kg. 

Note: the working above calculated the weight of water that is half filled. So he got confused and forgotten that the question is asking for the weight of the container. Other than that, that is a good model draw. Another interesting mistake is the working first calculated on the half filled water mass as 8.37 kg which is correct, but suddenly there is 1 more equal sign and the answer became 8.037. This type of mistake must be avoided. 

0 Replies 2 Likes ✔Accepted Answer

Container + 1/2(Water) = 12.14 kg

Container + Water = 20.51 kg

We can see the difference:

1/2 Water = 20.51 – 12.14

= 8.37 kg

Container = 12.14 – 8.37

= 3.77 kg

0 Replies 1 Like