To find the reciprocal of a fraction, flip it over, so that the numerator becomes the denominator and the denominator becomes the numerator. That is:
The reciprocal of 4/5 is 5/4
Note that the product of a fraction and its reciprocal is always 1.
4/5 x 5/4 = 20/20 = 1
In the case of a whole number, think of it as having a denominator of 1:
The reciprocal of 5 is 1/5.
5/1 x 1/5 = 5/5 = 1
Every number has a reciprocal except for 0. There is nothing you can multiply by 0 to create a product of 1, so it has no reciprocal.
Reciprocals are used when dividing fractions.