On Mr. Chernick's farm, they had three hens. The first laid one egg every day, the second laid one egg every other day, and the third laid one egg every third day. Mr. Chernick bought two new hens at the market, which also laid eggs completely regularly - always one after a few days, with one of them laying twice as often as the other.

Mrs. Chernick calculated that all five hens laid a total of 155 eggs in 60 days. How often did the new hens lay eggs?

(L. Hozová)
The first old hen laid one egg every day, i.e., 60 eggs in 60 days. The second old hen laid one egg every other day, i.e., 30 eggs in 60 days. The third old hen laid one egg every third day, i.e., 20 eggs in 60 days. The old hens laid a total of $60+30+20=110$ eggs in 60 days.

The old and new hens laid a total of 155 eggs in 60 days, so the new hens laid a total of $155-110=45$ eggs in 60 days. One of the new hens laid twice as many eggs as the other. By dividing 45 into three equal parts, we find that one new hen laid 15 eggs in 60 days, which means one egg every fourth day $(60: 15=4)$. The other new hen thus laid 30 eggs in 60 days, which means one egg every other day.

Suggested evaluation. 2 points for the total number of eggs laid by the three old hens in 60 days; 1 point for the total number of eggs laid by the two new hens in 60 days; 1 point for dividing the eggs from the new hens into three equal parts; 2 points for determining the laying rate of the new hens.