First, we need to add 2/6 and 2/5. 
2/6 can be simplified to 1/3
We need to have 1/3 and 2/5 have the same denominator. 3 and 5's LCM (least common multiply) is 15. 
Multiply both the numerator and denominator of 1/3 by 5. This equals 5/15. 
Multiply both the numerator and denominator of 2/5 by 3. This equals 6/15. 
5/15 plus 6/15 is 11/15. This means that Jack's 6000 dollars is 4/15.
Let's say that x is the total amount invested by all three. 
6000/x would equal 4/15. 
Solve this by multiply 6000 by 15 and 4 by x and making these two equal each other. 
90000 would equal 4x. Divide both sides by 4 to get x. 
90000 divided by 4 would be 22500. 
4/15 is the fraction by Jack 
22500 is the total amount invested by all 3