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.