28/12/2022

In Folha de S. Paulo, Marcelo Viana challenges readers.

Crédito: Freepik

Reproduction of Marcelo Viana's column in Folha de S. Paulo.

It's a tradition for this column to end the year by inviting readers to have fun with their families through math and logic challenges. I hope you enjoy them!

1. Ada and Bia like to play tennis, betting R$1 on each match. Ada won three bets, but ended up losing five reais. How many matches did they play?

2. While traveling, Malba Tahan comes to a fork in the road: in one direction is Mentirópolis, where everyone lies all the time; in the other is Verdadolândia, where everyone always tells the truth. Near the fork is a resident of one of the two cities, but we don't know which one. What question should Malba Tahan ask this person to find the way to Verdadolândia?

3. Every day, a ship leaves Rio de Janeiro for Lisbon and, at the same time, another ship leaves in the opposite direction, from Lisbon to Rio de Janeiro. I am on one of these ships, returning to Brazil, and I know that the trip lasts exactly 12 days and 12 nights in each direction. How many times, during the trip, will my ship cross paths at sea with others going in the opposite direction?

4. Three cowboys are standing in a line. Ado, the tallest, is at the back and sees the other two. Caio, the shortest, is in front and therefore sees no one. Beto is in the middle and only sees Caio. The three are close together and can hear each other. While they were blindfolded, hats were placed on their heads. They know there were three black hats and two white ones, but the remaining hats were taken from the spot before their blindfolds were removed. When asked, Ado replies that he doesn't know the color of his own hat. Then, Beto says he doesn't know the color of his hat either. Finally, Caio happily announces that he does know the color of his hat. What color is it, and how did he know?

Responses are welcome via email at viana.folhasp@gmail.com.

To read the full text, visit the newspaper's website.