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A tank filled with 200 litres of water weighs 350kg. The same tank filled with 150 litres of water weighs 315kg. What is the weight of the empty tank? 
Overseas Competition->Knowledge Point->Word Problem Modules->Applying Addition and Subtraction
T + 200L = 350kg $\cdots $ ①  -) T + 150L = 315kg $\cdots $ ②  \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_  (50L = 35kg) x3 = 150L = 105kg  315kg - 105kg = 210kg 