12. In the Century Park, there is a large lawn, where weeds grow every day (assuming the number of weeds growing per minute is fixed). Every morning at 8 o'clock, some workers remove the weeds (each person's weeding speed is the same), and once the weeds are removed (the number of weeds is 0, good grass is not removed), the workers finish their work, and the weeds that grow afterward are left for the next day. On the first day, some workers remove the weeds, finishing at 9 o'clock; on the second day, 10 workers remove the weeds, finishing at 8:30; on the third day, 8 workers remove the weeds, finishing at an integer hour (the final minute value is rounded to the nearest integer, fill in

$\qquad$
$\qquad$ the blank).
【Analysis】Let's assume that grass grows 1 unit per minute.
On the first day at 9:00, all the weeds on the entire grassland were cleared, i.e., the amount of grass was 0. Therefore, by 8:30 on the second day, the grass had grown for 23 hours and 30 minutes, which is 1410 minutes, totaling 1410 units of grass. These 1410 units of grass were cleared by 10 workers in 30 minutes,
so 1 worker can clear 1410 / 10 / 30 = 4.7 units of grass per minute.
On the third day at 8:00, the grass had grown for 23 hours and 30 minutes, which is 1410 minutes, totaling 1410 units of grass. 8 workers can clear 8 × 4.7 = 37.6 units of grass per minute, and it would take 1410 / (37.6 - 1) ≈ 39 minutes to clear the grass, i.e., they would finish work at 8:39 on the third day.