7. There are two strips of paper, one 21 cm long and the other 13 cm long. After cutting off the same length from both strips, it is found that the remaining length of the shorter strip is thirteen-eighths of the remaining length of the longer strip. How long was the piece that was cut off?
7.【Solution】The remaining length of the long paper strip: $(21-13) \div\left(1-\frac{8}{13}\right)=\frac{104}{5}=20.8$ (cm)

So the length of the cut segment: $21-20.8=0.2$ (cm)