City A is 25% less than population of city B. So city A's population is 75% of city B
Solution:
Given that City A is 25% less than population of city B
To find: city A's population is what % of city B
From given information,
City A is 25% less than population of city B
A = B - 25% of B
So we can write as,

A = 0.75B can be written as,

city A = 75 % of City B
Thus city A's population is 75% of city B