Answer
$60$
Work Step by Step
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 $y=6\sqrt {100}=60$