+118630 What number should be added to -3/5 to get 2/3?
Change the question so that the numbers are easy.
for example
What number should be added to 6 to get 10?
you can see that the answer is 4
you can also see that 10-6=4
so the answer is given by the 2nd number take away the first number.
so now do the same thing with your numbers
(2/3) - (-3/5) That will give you the answer 
What number should be added to -3/5 to get 2/3?
