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'Modular Fractions' printed from http://nrich.maths.org/

In arithmetic modulo 7 ($Z_7$) one integer is equal to another
if the difference between the two integers is a multiple of 7.
Rather like the days of the week, in $Z_7$ we only need seven
numbers and they are usually named 0, 1, 2, 3, 4, 5 and 6. 
If there is a solution in $Z_7$ to the equation $ax=1$ then we
call this solution the inverse (or reciprocal) of $a$ and write it
as $a^{1}$ or ${1\over a}$. For example the fraction one half in
arithmetic modulo 7 is the inverse of 2, that is the solution of
$2x=1 \pmod 7$, namely the number 4 because $2\times 4 = 1 \pmod
7$.
What are the fractions one third, one quarter, one fifth and
one sixth in arithmetic modulo 7?
Explain why all fractions in arithmetic modulo 7 are
equivalent to one of the following set of numbers $\{0, 1, 2, 3, 4,
5, 6\}$.
Show that in $Z_7$ there are six different solutions to the
equation
$${1\over x} + {1\over y} = {1\over {x+y}}.$$
Show that, by way of contrast, when working with real numbers
this equation has no real solutions.