Inside each of the circles that appear in the figure below, little Luana placed one of the numbers 1, 2, 3, 4, 5, 6, and 7.

![](https://cdn.mathpix.com/cropped/2024_05_01_cc59d970f26105cc4ca1g-13.jpg?height=416&width=483&top_left_y=503&top_left_x=889)

She did this in such a way that all the numbers were used. Her older brother, Pedro, looked at each trio of collinear circles and added the three numbers placed in them. Pedro observed that the sum was always the same.

a) Show that the number placed by Luana in the top circle is 4 and that the constant sum observed by Pedro is equal to 12.

b) Find a way in which Luana could have managed to achieve this feat.