Jenn ate \( \frac{3}{10} \) of the candy bar, and David ate \( \frac{4}{10} \) of the candy bar.
First, we can add how much they both ate together:
\[ \frac{3}{10} + \frac{4}{10} = \frac{7}{10} \]
To find out how much candy bar is left, we subtract the amount they ate from the whole candy bar (which is represented as 1):
\[ 1 - \frac{7}{10} = \frac{10}{10} - \frac{7}{10} = \frac{3}{10} \]
So, the amount of candy bar left is \( \frac{3}{10} \).