The standard way is to draw a Venn diagram like the one that I've attached.
You'll see that I have drawn two overlapping loops representing the students who take Japanese (J) and government (G). Since we don't know how many take both, write in the overlap. Then work outwards:There are altogether in J; we've placed of them in the overlap, so there must be left in the other part of the loop.Then add up the numbers in each of the four regions, putting the total equal to :
In the same way, there are in the right-hand part of loop G.
There are who aren't in either loop. These go outside the loops, then.