The hammer rang once. A spark jumped off the anvil and did not go out. It waited. They followed.

Beto had the ring with one brass key on it. Inkling had a pencil. Danya had questions.

The spark rolled down the lane behind the print shop, and the lane became a street of shops. Every shop sold something to count with. Abacuses, scales, ledgers with red covers, chalk by the box.

A sign said TALLY ROW.

The street was having a quiet sort of fuss. A table stood in the middle with a cash box and a ledger. The ledger's spine had come apart long ago and been sewn back with string. Beside the stitches, somebody had drawn a tiny hammer in pencil.

"The fair made three hundred and ninety-three," said the bread seller. "That's what the ledger says."

"And the cash box?" asked Danya.

The cake seller opened it. Six pound coins and some silver. Six pounds ninety.

"So money has gone missing," said the pie seller. She looked at the cake seller. The cake seller looked at the floor.

"I can name it," said Inkling. "The cake stall took it. They opened the box."

The cake seller went pale.

"Put it back in and check," said Vetta.

A heron in reading glasses stood behind the table. She held the ledger close to her beak.

"Does the answer fit the question?" said Vetta. "Look back before you blame. Let's check that line again."

Inkling felt her cheeks burn. "Sorry," she said to the cake seller. "I said it too soon."

"You did," said the cake seller. "Thank you for saying sorry."

A ferret came running with a fistful of paper tags. He tied one to the cash box and one to the table leg.

"Read me the lines," said Tagg.

Beto read them. "Bread, 240. Cakes, 3. Pies, 150."

"Three what?" said Tagg. Then he sat on the cash box and said nothing at all.

Beto frowned. He went to the cake seller. "Three what?" he asked her.

"Pounds," she said. "Three pounds. I always write it in pounds. It's shorter."

He went to the bread seller. "Pennies." He went to the pie seller. "Pennies."

"So this column has two kinds of number in it," said Beto. "Pennies and pounds, mixed up. Three pounds is three hundred pennies."

"A number without a unit," said Tagg, "is a mystery."

Try the Packet now, if you have it. Then read on.

A raccoon with chalk on his paws knelt by the table.

"Let's make it a bar," said Doodle.

He drew a long bar on the cobbles for the whole fair and split it into three parts. Then he held out the chalk to Inkling.

Inkling took it. After the cake stall, her hand was not quite steady.

"All in pennies," she said. "Bread is 240." She chalked it under the first part. "Cakes are three pounds, so 300." She chalked that under the middle part. "Pies are 150."

The cake part was the biggest. That looked right. Three pounds is a lot of cake.

"Now the whole," said Doodle.

Inkling added. "Two hundred and forty and three hundred is five hundred and forty. And a hundred and fifty. Six hundred and ninety."

"That's six pounds ninety," said Beto. "That's what's in the box."

"How do you know you added it right?" asked Vetta.

Nobody answered.

"What if you added it the other way round?" she said, and folded her wings.

Danya took the chalk. He started with the pies. "One hundred and fifty. And three hundred makes four hundred and fifty. And two hundred and forty makes six hundred and ninety."

"The same," said Danya. "So the ledger was wrong, not the box. Nothing was missing. Three pounds was written as three."

Danya took the pencil. He wrote the ledger out again, with a little p or a little £ beside every number.

"How do we know the next person won't do it again?" he asked. "Because now they can check."

The cake seller gave them a copper key from Tally Row. "For the ring," she said. "And for asking before blaming."

Beto put it beside the brass one.

The spark rolled back up the lane, and the lane was just the lane behind the print shop.

Back on the Green

The spark dimmed to a warm dot and settled back on the anvil. The Green was just the Green again.

Beto's dad said the shopping came to "four and a half." Beto asked, "Four and a half what?" It was pounds. The receipt said so.