## The Challenge

All that planning has earnt you a break, so you decide to take a nap at the library. As you make your way there, you overhear some cops discussing when the last secret bank-heist occurred in the city.

Because you’re a nosey little thief, you manage to pickpocket 2 pieces of paper from them. You need to use the 2 notes to give you the information of the date of the last bank-heist…

When was the last bank-heist?

## The Solution

### The puzzle bit

The challenge is called *The Library *and upon closer inspection of the golden note, you may notice that the first letter of each word cycles through the letters ISBN: ISBN = International Standard Book Number!

### The algebra bit

The 2nd note has a string of letters and numbers, but we do not know what the letters stand for. However we have some clues to solve them…

**What is A?**

The first equation (K + D)^{2} = A shows us that A must be a square number. The only possibilities between 1-9 are 1,4, or 9.

K and D must be also be between 1-9 and they must be *different*. The smallest possible choice here is that K and D are equal to 1 and 2 in some order, but A=1 or A=4 are too small to accept even the smallest valid values for K+D. Therefore A=9 which implies that K + D = 3 (incidentally also the smallest possible sum).

**What are K and D?**

Since we know that K+D=3, we have that either:

- K=1 and D=2 or;
- K=2 and D=1

If K=1, by the second equation C/F = 1, but this means that C=F which is a contradiction, since each letter must represent a different digit. Therefore, K=2 and D=1.

**What is H?**

Using the 3rd equation, with A=9 and K=2, we see that A-H=K implies that H=7.

**What is C?**

By the 4th equation: H+D=C, with H=7, D=1, we have that C=8

**What is F?**

By the 2nd equation: C/F = K, with C=8, K=2 we have that F=4.

**Conclusion:**

Now that we have found what each letter represents, we can see that the string of numbers is *9781444727296*! This is the ISBN number for the book 11/22/63 (Stephen King) which is the date of the last bank heist! (Apologies for the Americanness of the date).