John is cooking cupcakes. The recipes calls for 3 3/10

To determine how many extra cups of flour John added, we need to subtract the amount of flour the recipe called for from the amount he actually used.

The recipe calls for \(3 \frac{3}{10}\) cups of flour. This can be converted to an improper fraction:

\[
3 \frac{3}{10} = \frac{30 + 3}{10} = \frac{33}{10}
\]

John accidentally used \(3 \frac{5}{8}\) cups of flour, which can also be converted to an improper fraction:

\[
3 \frac{5}{8} = \frac{24 + 5}{8} = \frac{29}{8}
\]

Now, we will find a common denominator to subtract these two fractions. The least common multiple of the denominators 10 and 8 is 40.

Now convert both fractions:

\[
\frac{33}{10} = \frac{33 \times 4}{10 \times 4} = \frac{132}{40}
\]

\[
\frac{29}{8} = \frac{29 \times 5}{8 \times 5} = \frac{145}{40}
\]

Now, we can subtract:

\[
\frac{145}{40} - \frac{132}{40} = \frac{145 - 132}{40} = \frac{13}{40}
\]

Thus, John put in \( \frac{13}{40} \) extra cups of flour.

The answer is:

**13/40**