the table above shows how I would set up the problem. Put <6 / 6+ in the columns, tired/no tired in rows, add a total row and column. the basic premise is that you can add down every column and across every row.
I plugged in 100 for the total/total for convenience. Note that the 70% is taken from the interns who sleep more than 6 (x in the total/6+ box), not from the total/total number of interns. (that's probably where you went wrong in your solution)
Once you have the table set up, mark the target box (total / no tired), and work your way to it with the steps below the table. Answer is 19.
