Answer:
Make a Venn diagram for this
it would look something like this
So this means you have to find the intersection of both the subjects
and you know that if you do 50 - econ union government = 7
as those students take neither
so just minus 17 - 7 = 10
30 -7 = 23
and add 23 +10 = 33, 50 - 33 = 17 so 17 take both the subjects
See the diagram it will be clear i hope
