Meu pai tem 7 filhos (é verdade). Em um determinado ano, quando éramos crianças, ele nos levou ao parque, em grupos de 3, a cada sábado, sem nunca repetir o mesmo grupo. Cada trio possível foi ao parque com ele exatamente uma vez. Quantas vezes eu fui ao parque durante aquele ano?
(My father has 7 children (it's true). In a certain year, during our childhood, he took us to the park, in groups of 3, each Saturday, never repeating the same group. Every possible trio went to the park with him exactly once. How many times did I go to the park during that year?)
PCFilho
15 vezes
ResponderExcluirCerto! :-)
ExcluirPC, seu blog tem página no Facebook?
ResponderExcluirNão... mas se quiser me adicionar, meu perfil é esse aqui.
ExcluirSince the trio is different every time, the number of times you went to the park is equal to the number of total possible trios minus the ones that don't include you. The total number of possible trios is the number of different ways that 3 things can be selected from a set of 7, which is (7*6*5)/(3*2*1) = 35. The number of possible trios that don't include you is the number of different ways that 3 things can be selected from a set of 6, which is (6*5*4)/(3*2*1) = 20. Therefore, the number of times you went to the park that year is 35 - 20, or 15.
ResponderExcluirPerfect explanation, Jake. :)
Excluir