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# Puzzles

## Archive

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## Four points on a shape

Fiona wants to draw a 2-dimensional shape whose perimeter passes through the points A, B, C and D
Which of the following shapes can she draw?
(i) A circle
(ii) An equilateral triangle
(iii) A square

## Algebraic fractions

Given that
$$\frac{3x+y}{x-3y}=-1$$
what is the value of
$$\frac{x+3y}{3x-y}$$
?
Tags: algebra

## Four integers

$$a$$, $$b$$, $$c$$ and $$d$$ are four positive (and non-zero) integers.
$$abcd+abc+bcd+cda+dab+ab+bc+cd+da+ac+bd\\+a+b+c+d=2009$$
What is the value of $$a+b+c+d$$?

## Sum

What is
$$\sum_{i=1}^{\infty}\frac{1}{i 2^i}$$
?

## Fair dice

Timothy and Urban are playing a game with two six-sided dice. The dice are unusual: Rather than bearing a number, each face is painted either red or blue.
The two take turns throwing the dice. Timothy wins if the two top faces are the same color, and Urban wins if they're different. Their chances of winning are equal.
The first die has 5 red faces and 1 blue face. What are the colours on the second die?

## Half digits

Source: Maths Jam
Can you use each of the digits 1 to 9 to make a fraction which is equal to a half?
Tags: numbers

## Pizza

Twelve friends want to share a pizza. One of the friends is very fussy and will not eat the centre of the pizza.
Is it possible to split a (circular) pizza into twelve identical pieces such that there is at least one piece which does not touch the centre?

Source: nrich