she needs in total 40,000 votes, and from those 40,000, 4/5 need to be from senior citizens, now, 5/5 is a "whole", so 5/5 - 4/5 that leaves only 1/5.
therefore, of those 40,000 votes, 1/5 can be of non-senior citizens, how much is 1/5 of 40,000? well, is just their product,
[tex]\bf \cfrac{1}{5}\cdot 40000\implies \cfrac{40000}{5}\implies 8000[/tex]
