Cecilia and her dad are climbing stairs from the first floor. They start at the same time, and each of them keeps her or his speed of climbing unchanged. When Cecilia arrives at the third floor, her dad arrives at the fifth floor. When Cecilia arrives at the eighth floor, which floor does her dad arrive at? 
Overseas Competition->Knowledge Point->Word Problem Modules->Interval Problems
$(5-1) \div(3-1)=2$. $(8-1) \times 2+1=15$. 