Answer
College B
Work Step by Step
Let $x$ be the population in College $A \&$ y be the population in College $B$
This means that there is an $81 \%$ chance of picking either of the 3 in $x+y$ (the total population) and is equal to the sum of percentage of the 3 in each college:
$81 \%(x+y)=85 \%(x)+80 \%(y)$
$0.81(x+y)=0.85 x+0.80 y$
$0.81 x+0.81 y=0.85 x+0.80 y$
$0.01 y=0.04 x \quad \leftarrow$ Divide by $0.01 x$
$\frac{y}{x}=\frac{4}{1}$
This corresponds to the ratio of the population in College B to College A which means College B has more undergraduates.
