The friction of the air causes a vertical retardation equal to 10% of the acceleration due to gravity `(take g=10ms^(-2))` The maximum height will be decreased by:

Explanation

To solve the problem, we need to determine how the maximum height of a projectile is affected by the air friction, which causes a vertical retardation equal to 10% of the acceleration due to gravity.

### Step-by-Step Solution:

1. **Understanding the Problem**:
- The maximum height \( h \) of a projectile without air resistance is given by the formula:
\[
h = \frac{u^2 \sin^2 \theta}{2g}
...