There are many ways to understand this problem. You are told that 5/6th of the students take a FL. So if you think of this as pie chart there will be one big piece (5/6) and one small piece (1/6). What you need to do now is figure out how much 2/5 is of 5/6 that is 2/5x5/6=10/30 or 1/3 take French
Another way to do this is to suppose the total number of six grade students is say 300 then 300x5/6=250 take a FL and of those 250 2/5 or 100 take French. Now to answer the question we must compute the fraction of students who take French that is 100/300=1/3.
Hope this helps..let me know if you have any questions.