Two partners invested a total of 45,000 dollars and one of the partners, John invested 4/9 of the amount which 45000. That means John's own contribution is four-ninths of the total amount and that can be derived as follows;
John = 4/9 * 45000
John = (4 * 45000)/9
John = 180000/9
John = 20000
John's investment therefore is $20,000