其它
A tank filled with $200$ litres of water weighs $350\rm{kg}$. The same tank filled with $150$ litres of water weighs $315\rm{kg}$. 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 