On a 12 × 12 grid, some squares are infected and some are healthy. On each turn, a healthy square becomes infected if it has two or more infected orthogonal neighbors. (In the example above, the black squares are infected, the white squares are healthy, and the gray squares will be infected next turn.) What’s the smallest number of initially infected squares that can spread an infection over the whole board?

# Puzzles

# Counterfeit Redux

A more challenging version of the Counterfeit Coin puzzle from 2011:

You have 12 coins, one of which has been replaced with a counterfeit. The false coin differs in weight from the true ones, but you don’t know whether it’s heavier or lighter. How can find it using three weighings in a pan balance?

# “A Financial Puzzle”

This, now, is straightforward and business-like: A. applied to B. for a loan of $100. B. replied, ‘My dear A., nothing would please me more than to oblige you, and I’ll do it. I haven’t $100 by me, but make a note and I’ll indorse it, and you can get the money from the bank.’ A. proceeded to write the note. ‘Stay,’ said B., ‘make it $200. I want $100 myself.’ A. did so, B. indorsed the paper, the bank discounted it, and the money was divided. When the note became due, B. was in California, and A. had to meet the payment. What he is unable to cipher out is whether he borrowed $100 of B., or B. borrowed $100 of him.

— Henry C. Percy, *Our Cashier’s Scrap-Book*, 1879

# Black and White

By Jan Hartong. White to mate in two moves.

# Caller ID

In his 1936 collection *Brush Up Your Wits*, British puzzle maven Hubert Phillips relates that his brother-in-law felt himself cursed with an unintelligent maid. “I have just overheard her taking a ‘phone call,” he told Phillips, “and this is what I heard:

“‘Is Mr. Smith at home?’

“‘I will ask him, sir. What name shall I give him?’

“‘Quoit.’

“‘What’s that, sir?’

“‘Quoit.’

“‘Would you mind spelling it?’

“‘Q for quagga, U for umbrella, O for omnibus, I for idiot –‘

“‘I for what, sir?’

“‘I for idiot, T for telephone. Q, U, O, I, T, Quoit.’

“‘Thank you, sir.'”

Why did he accuse her of unintelligence?

# Brood War

A newlywed couple are planning their family. They’d like to have four children, a mix of girls and boys. Which is more likely: (1) two girls and two boys or (2) three children of one sex and one of the other? (Assume that each birth has an equal chance of being a boy or a girl.)

# Black and White

By William Meredith. White to mate in two moves.

# Flip Sum

A problem from the 1999 St. Petersburg City Mathematical Olympiad:

Fifty cards are arranged on a table so that only the uppermost side of each card is visible. Each card bears two numbers, one on each side. The numbers range from 1 to 100, and each number appears exactly once. Vasya must choose any number of cards and flip them over, and then add up the 50 numbers now on top. What’s the highest sum he can be sure to reach?

# Black and White

By Peder Andreas Larsen. White to mate in two moves.

(I’ve added a guide to chess notation.)

# Black and White

By Francis Healey. White to mate in two moves.