Three investors own a company. one partner owns 1/4 of the company , while the second partner owns 2/5 of the company . how much does the third partner own ?

We want to add these together and then subtract from one. So to add them, find a common denominator.
The closest common denominator is 20.
So convert the fractions:
5/20 and 8/20
Then add
5/20 + 8/20 = 13/20
And then subtract from 1.
1 - 13/20 = 7/20
The third partner owns 7/20 of the company.