Let's denote the additional fee Jackson charges for each car wash as \( C \) dollars.
From the given information, we can set up two equations based on the services he provided:
1. The total income from mowing lawns: \( 5 \times 15 = 75 \) dollars.
2. The total income from washing cars: \( 10 \times C \) dollars.
The total income he made is $90. So, we can write the equation:
\[ 75 + 10C = 90 \]
Now, solve for \( C \):
\[ 10C = 90 - 75 \]
\[ 10C = 15 \]
\[ C = 1.5 \]
Jackson charges $1.5 for each car wash.