# Difference between revisions of "Euler problems/151 to 160"

## Problem 151

Paper sheets of standard sizes: an expected-value problem.

Solution:

```problem_151 = undefined
```

## Problem 152

Writing 1/2 as a sum of inverse squares

Note that if p is an odd prime, the sum of inverse squares of all terms divisible by p must have reduced denominator not divisible by p.

Solution:

```import Data.Ratio
import Data.List

invSq n = 1 % (n * n)
sumInvSq = sum . map invSq

subsets (x:xs) = let s = subsets xs in s ++ map (x :) s
subsets _      = [[]]

primes = 2 : 3 : 7 : [p | p <- [11, 13..83],
all (\q -> p `mod` q /= 0) [3, 5, 7]]

-- All subsets whose sum of inverse squares,
-- when added to x, does not contain a factor of p
pfree s x p = [(y, t) | t <- subsets s, let y =  x + sumInvSq t,
denominator y `mod` p /= 0]

-- Verify that we need not consider terms divisible by 11, or by any
-- prime greater than 13. Nor need we consider any term divisible
-- by 25, 27, 32, or 49.
verify = all (\p -> null \$ tail \$ pfree [p, 2*p..85] 0 p) \$
11 : dropWhile (< 17) primes ++ [25, 27, 32, 49]

-- All pairs (x, n) where x is a rational number whose reduced
-- denominator is not divisible by any prime greater than 3;
-- and n>0 is the number of sets of numbers up to 85 divisible
-- by a prime greater than 3, whose sum of inverse squares is x.
only23 = foldl f [(0, 1)] [13, 7, 5]
where
f x p = collect \$ concatMap (g p) x
g p (x, n) = map (\(a, b) -> (a, n * length b)) \$ pfree (terms p) x p
terms p = [n * p | n <- [1..85`div`p],
all (\q -> n `mod` q /= 0) [5, 7, 11, 13, 17]]
collect = map (\z -> (fst \$ head z, sum \$ map snd z))
. groupBy cmpFst . sort
cmpFst x y = fst x == fst y

-- All subsets (of an ordered set) whose sum of inverse squares is x
findInvSq x y = f x \$ zip3 y (map invSq y) (map sumInvSq \$ init \$ tails y)
where
f 0 _        = [[]]
f x ((n, r, s):ns)
| r > x     = f x ns
| s < x     = []
| otherwise = map (n :) (f (x - r) ns) ++ f x ns
f _ _        = []

-- All numbers up to 85 that are divisible only by the primes
-- 2 and 3 and are not divisible by 32 or 27.
all23 = [n | a <- [0..4], b <- [0..2], let n = 2^a * 3^b, n <= 85]

problem_152 = if verify
then sum [n * length (findInvSq (1%2 - x) all23) |
(x, n) <- only23]
else undefined
```

## Problem 153

Investigating Gaussian Integers

Solution:

```problem_153 = undefined
```

## Problem 154

Exploring Pascal's pyramid.

Solution:

```problem_154 = undefined
```

## Problem 155

Counting Capacitor Circuits.

Solution:

```problem_155 = undefined
```

## Problem 156

Counting Digits

Solution:

```problem_156 = undefined
```

## Problem 157

Solving the diophantine equation 1/a+1/b= p/10n

Solution:

```problem_157 = undefined
```

## Problem 158

Exploring strings for which only one character comes lexicographically after its neighbour to the left.

Solution:

```problem_158 = undefined
```

## Problem 159

Digital root sums of factorisations.

Solution:

```problem_159 = undefined
```

## Problem 160

Factorial trailing digits

Solution:

```problem_160 = undefined
```