11. (5 points) Place 100 ping-pong balls into 26 boxes arranged in a row from left to right. If the leftmost box contains 4 ping-pong balls, and the sum of the number of ping-pong balls in any 4 consecutive boxes is 15, then the number of ping-pong balls in the rightmost box is $\qquad$.
【Solution】According to the analysis, 26 boxes are divided into: $26 \div 4=6$ (groups) $\cdots 2$ (boxes).
$\because$ In any 4 adjacent boxes, the total number of ping-pong balls is 15, so the number of balls in the boxes at positions $1,5,9 \cdots 25$ are all 4.

The number of ping-pong balls in the rightmost box: $100-(15 \times 6+4)=6$ (balls).
The answer is: 6