To determine the cost of each mechanical pencil, let's denote the cost of each mechanical pencil as \( x \).
Mr. Spears is buying bundles of mechanical pencils and erasers, and the total cost of the mechanical pencils and erasers must add up to $25. Since erasers come in bundles of 10 and each eraser costs $0.50, the cost for one bundle of 10 erasers would be:
\[ 10 \times 0.50 = 5 \]
Now, if Mr. Spears buys \( y \) bundles of mechanical pencils, the equation representing the total cost becomes:
\[ yx + 5 = 25 \]
However, for the equation you've provided, we'll focus just on the cost of the mechanical pencils and the cost of the erasers.
If we consider he might be purchasing 10 erasers, we have:
\[ x + 0.50 = 25 \]
This implies that \( x \), the total cost of mechanical pencils must be equal to \( 25 - 0.50 \):
\[ x + 0.50 = 25 \]
So, in this case, where \( x \) represents the total cost of mechanical pencils, the equation would be:
\[ x + 0.50 = 25 \]
Thus, the cost of each mechanical pencil (if he buys a certain number of them) leads us to fill:
\[ \textbf{(x + 0.50)} \]
So, the completed equation with the missing place filled in becomes:
\[ x + 0.50 = 25 \]
Thus, the cost of mechanical pencils would be \( x = 25 - 0.50 = 24.50 \). Each mechanical pencil's cost will depend on how many he buys.