What was the question for this implicit rationale, and corresponding answer?
The relevant information is: The Treaty of Tordesillas (Portuguese: Tratado de Tordesilhas ( ttau tuzi), Spanish: Tratado de Tordesillas (tatao e toesias) ), signed at Tordesillas on June 7, 1494, and authenticated at Setubal, Portugal, divided the newly discovered lands outside Europe between the Portuguese Empire and the Crown of Castile, along a meridian 370 leagues west of the Cape Verde islands, off the west coast of Africa.
The answer: The Treaty of Tordesillas ( Portuguese : Tratado de Tordesilhas ( ttau tuzi ) , Spanish : Tratado de Tordesillas ( tatao e toesias ) )
The question: The Treaty of Tordesillas (Portuguese: Tratado de Tordesilhas ( ttau tuzi), Spanish: Tratado de Tordesillas (tatao e toesias) ), signed at Tordesillas on June 7, 1494, and authenticated at Setubal, Portugal, divided the newly discovered lands outside Europe between the Portuguese Empire and the Crown of Castile, along a meridian 370 leagues west of the Cape Verde islands, off the west coast of Africa. This line of demarcation was about halfway between the Cape Verde islands (already Portuguese) and the islands entered by Christopher Columbus on his first voyage (claimed for Castile and Leon), named in the treaty as Cipangu and Antilia (Cuba and Hispaniola).

Answer this question: spain and portugal divide the world between them in?