Problem 5. Biljana, Maria, and Elena often go to the library. Biljana goes every 3 days, Maria every 8 days, and Elena every 15 days. On which next date will they all meet at the library, if they previously met there on April 11?
Solution. The least common multiple of the numbers 3, 8, and 15 is 120. This means that all three will meet again in the library after 120 days. Considering that April and June have 30 days, and May and July have 31 days, and they met on April 11, they will meet again on $11+120-(2 \cdot 30+2 \cdot 31)=131-122=9$th August.

## VI Section