A library has an average of 252 visitors on Sundays, and 120 on other days.…

When a set of items splits into two groups with different per-unit values, the overall average is a weighted average: multiply each group's value by its count, add the two products, and divide by the total count -- not a simple average of the two rates.

1. June has 30 days in total.

2. Since June begins on a Sunday, the Sundays in the month fall on the 1st, 8th, 15th, 22nd, and 29th -- 5 Sundays.

3. The remaining 30 − 5 = 25 days are the "other days" with the lower visitor count.

4. Total visitors on Sundays = 5 × 252 = 1260.

5. Total visitors on the other days = 25 × 120 = 3000.

6. Total visitors for the whole month = 1260 + 3000 = 4260.

7. Average visitors per day = 4260 ÷ 30 = 142.

As an independent check, start from the lower rate and add only the extra contributed by Sunday's higher rate, weighted by how often Sundays occur: 120 + (5/30) × (252 − 120) = 120 + (1/6) × 132 = 120 + 22 = 142, which matches the direct calculation.

So the average number of visitors per day in June is 142.