Your privious task:
Before I start writing my answer I would like to mention that I don't speak English very well, so I am sorry if I make any mistakes (speaking, not maths xD)
So lets begin. I'd solve this task by making a system. Lets say he was in Portugal for x months and in Brazil for y months. Then x+y=14 and 130x+150y=1920:
[tex]\left \{ {{x+y=14} \atop {130x+150y=1920}} \right. \\\left \{ {{y=14-x} \atop {13x+15(14-x)=192}} \right. \\\text{Lets solve the 2nd line:}\\ 13x+15(14-x)=192\\2x=18\\x=9\\\text{Then y=5}[/tex]
So he lived in Portugal for 9 months and in Brazil for 5 months.
Solution for this task: x=-22/83 and y=-81/83 (c is right)
