smc

Translating each sentence into an equation, $a=1.5c$ and $b=1.25c$. We want a relationship between $a$ and $b$. Dividing the second equation into the first will cancel the $c$, so we try that and get: $\frac{a}{b}=\frac{1.5}{1.25}$ $\frac{a}{b}=\frac{150}{125}$ $\frac{a}{b}=\frac{6}{5}$ $a=1.2b$ In this case, $a$ is $1.2-1=0.2=20\%$ bigger than $b$, and the answer is $\boxed{20\%}$.