8. There is a pair of tightly adjacent rubber drive wheels, each with a mark line passing through the center (as shown in the figure below).

The radius of the driving wheel is 105 cm, and the radius of the driven wheel is 90 cm. When they start to rotate, the mark lines on both wheels are in a straight line. Question: How many turns does the driving wheel need to make at least so that the mark lines on both wheels are in a straight line again?
8. The driving gear has rotated 3 turns
8.【Solution】The least common multiple of 105 and 90 is $630. 630 \div 105=6$,

So the driving gear has rotated 6 half-turns, which is 3 turns, and the marks on both gears are aligned again