and B start from two places 100 kilometers apart at the same time and walk toward each other. A travels 10 kilometers per hour and B travels 15 kilometers per hour. A dog with A travels 20 kilometers per hour. The dog starts with A, turns around and runs towards A when encountering B, and runs like this repeatedly until the two meet. How many miles did the dog run in total?
甲乙两人同时从相距100千米的两地出发，相向而行，甲每小时10千米，乙每小时行15千米．甲带着的一条狗，每小时行20千米．这只狗同甲一道出发，碰到乙掉头朝甲跑去，再如此反复跑动，直到两人相遇为止．这只狗一共跑了多少千米？
<gadget id="calculator">10 + 15</gadget>
<output>25</output>

<gadget id="calculator">100 / 25</gadget>
<output>4</output>

<gadget id="calculator">20 * 4</gadget>
<output>80</output>

<result>80</result>