Chapter 3 - Polynomial and Rational Functions - Test - Page 404: 21

$60$

Step 1. Since y varies directly as the square root of x, we can write $y=c\sqrt x$ where $c$ is a constant. Step 2. As $y=12$ when $x=4$, we have $12=c\sqrt 4$, thus $c=6$ Step 3. For $x=100$, we have $y=6\sqrt {100}=60$