Mary ate \( \frac{3}{10} \) of the cookie, and Peter ate \( \frac{4}{10} \) of the cookie. To find out how much of the cookie is left, we first add the portions that Mary and Peter ate:
\[ \frac{3}{10} + \frac{4}{10} = \frac{3 + 4}{10} = \frac{7}{10} \]
This means that together, they ate \( \frac{7}{10} \) of the cookie.
To find out how much is left, we subtract the amount eaten from the whole cookie (1):
\[ 1 - \frac{7}{10} = \frac{10}{10} - \frac{7}{10} = \frac{3}{10} \]
Therefore, the amount of cookie that is left is \( \frac{3}{10} \).