Girls were collecting caps from PET bottles. Šárka collected 20, Světlana 29, Marta 31, Maruška 49, and Monika 51. Each of the girls put all the caps they collected either into a blue or a red box. Pavlík, while counting the caps, found out that there were twice as many caps in the blue box as in the red one. Which of the girls put their caps into the blue and which into the red box?

(L. Hozová)
First, we add up all the bottle caps the girls collected, i.e., $20+29+31+49+51=180$. Since the blue box should contain twice as many bottle caps as the red box, we need to divide 180 bottle caps into 60 and 120, i.e., 1 part and 2 parts. Then we look for which two of the five numbers add up to 60. The only option is $60=29+31$, and then $120=20+49+51$. Therefore, the bottle caps in the red box were collected by Svetlana and Marta, and the bottle caps in the blue box were collected by Sarka, Maruska, and Monika.