Next door to the Li family was another household — also a mud house with an earthen yard, deeply impoverished.

Generally speaking, aside from the old scholar’s stately mansion, only the village head’s home was presentable. Every other household was poorer than the last.

An old woman sat in a dusty, gray courtyard, holding a long-stemmed pipe, puffing away on it with a pock-pock sound.

The smell was acrid — almost certainly the coarsest, cheapest tobacco leaves.

Noticing the scrutinizing gaze from outside the gate, the old woman lazily lifted her eyelids, swept a disinterested glance their way, then resumed smoking her pipe, paying them no further mind.

“It seems most of the people in this village are elderly,” Bai Youwei murmured quietly to Shen Mo.

Whether it was the old scholar, or the village head, or Ma-shi — all were people well past sixty.

Even the village head’s son and daughter-in-law they had seen at the mourning hall appeared to be in their early forties. And beyond that, there were only young children. There were notably no teenagers or people in their twenties.

“This place is both out of the way and poor — what young person would stay here?” The old woman in the courtyard suddenly spoke up. “Those who could leave have gone. The ones left behind are just the old folks who can’t move anymore, and the old bachelors who could never afford a wife.”

Everyone startled slightly. They hadn’t expected this NPC to volunteer information so readily.

Du Lai stepped into the yard and asked politely: “Do you know the Li-shi from next door?”

The old woman heard this and smiled without a sound. The wrinkles on her face were like deep, layered ravines, giving one an extraordinarily uneasy feeling.

At that moment, a series of coughs came from inside the room, and then they heard an old man say:

“I’ve been ill for a long time now. When will my daughters come to visit me?”

The old woman held her pipe and gave it a light knock against the ground, then said to the old man inside: “Didn’t your eldest come to see you just yesterday?”

“Why can’t I see the three of them come visit together?”

The old woman snorted coldly and said: “That’s no easy matter — the eldest comes every five days, the second comes every four days, and the youngest comes every three days. Who knows when they’ll all arrive together to see you.”

As she spoke, her gaze drifted over to Du Lai, and to Fu Miaoxue, Shen Mo, and Bai Youwei standing behind him.

Everyone understood at once: the NPC was presenting them with a puzzle.

Bai Youwei felt a measure of relief.

Good — this problem isn’t difficult.

She asked the old woman: “When was the last time all three daughters were here together?”

“They gathered once for the Mid-Autumn Festival,” the old woman answered.

“How many days have passed since the Mid-Autumn Festival?” Bai Youwei asked again.

The old woman gave a cold little laugh: “I’m old — I can’t remember clearly…”

Bai Youwei’s brow furrowed slightly.

Not just her — Shen Mo and Du Lai also frowned.

“What’s wrong?” Fu Miaoxue asked, not understanding. “Is the problem difficult?”

“The problem itself isn’t hard, but it’s missing a piece.” Bai Youwei explained: “This is a problem from the ancient *Sunzi Calculation Classic*. It asks: a family has three daughters, all married out. The eldest daughter returns home every five days, the second daughter every four days, the youngest every three days. After all three daughters leave home on the same day, how many days until all three meet together again?”

“How many days?” Fu Miaoxue blurted.

Bai Youwei: “Sixty days.”

The number of days is the least common multiple of the three return intervals.

That is, 3 × 4 × 5 = 60.

Fu Miaoxue blinked and said: “Then just tell her the answer.”

Du Lai took her hand and pulled her behind him. “What’s missing is the initial condition. We don’t know which day all three daughters left together.”

“There should be a clue nearby about the Mid-Autumn Festival,” Shen Mo said. “Let’s split up and look.”

As long as they figured out which day the Mid-Autumn Festival fell in this game, they could calculate when all three daughters would next be home at the same time.

Fu Miaoxue said: “Why go to all that trouble~”

And with that, she bounced over in front of the old woman: “Hey, your three daughters are coming back tomorrow~”

—