109. What is the last digit of the difference

$$
1 \cdot 2 \cdot 3 \cdot 4 \ldots 13-1 \cdot 3 \cdot 5 \cdot 7 \ldots 13 ?
$$
109. The minuend ends in 0, and the subtrahend ends in 5. Therefore, the difference ends in 5.