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Here's the official property:
 

The Multiplicative Inverse Property:

The
multiplicative inverse of a number,
 a is1/aso that

a x 1/a = 1
 
and

 
1/a x a = 1

This is often written in one line...

a x 1/a = 1 = 1/a x a

Where a is any real number.

So, how do we find the multiplicative inverse of something like

2/5

Easy!  Just flip it!

2/5 x 5/2 = 10/10 = 1

So, the multiplicative inverse of 2/5 is 5/2.