In a certain computer company, 3/5 of the programmers know fortran, 1/6 know cobol, and 1/10 know both. what fraction know neither fortran nor cobol?

Given that 3/5 know fortran
1/6 know cobol
1/10 know both fotran and cobol
the fraction of programmers who knows at least one of the two programs will be:
3/5+1/6+1/10
=13/15
the fraction of programmers who don't know either of the two will be:
1-13/15
=2/15
thus the answer is 2/15