[ Percentage and ratio problems ]

Waking up every morning at 8:30, the stoker fills the stove with coal to the brim. At this time, he puts in exactly 5 kg of coal. Every evening, when he goes to bed (and he goes to bed at the same time every day), he again fills the stove with coal to the brim and puts in exactly 7 kg of coal.

At what time does the stoker go to bed?
Since in the morning 5 kg of coal fits into the stove, and in the evening - already 7 kg, then during the time the stoker is awake, 7 kg of coal burns (5 kg that were put in the morning, and another 2 kg that were already in the stove). In the evening, 7 kg of coal is put into the stove, so during the time the stoker is sleeping, 5 kg of it burns (so that in the morning exactly 5 kg of coal can be put in).

Thus, the moments of falling asleep and waking up of the stoker divide the day into two parts, the lengths of which are in the ratio of $7: 5$, that is, into 14 and 10 hours. Therefore, the stoker is awake for 14 hours.

Answer

B 22.30