T + 200L = 350kg $\cdots $ ①  -) T + 150L = 315kg $\cdots $ ②  \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_  (50L = 35kg) x3 = 150L = 105kg  315kg - 105kg = 210kg 
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? 

a. $$120\rm{kg}$$ 
b. $$150\rm{kg}$$ 
c. $$165\rm{kg}$$ 
d. $$210\rm{kg}$$ 
e. None of the above 