This question is not difficult, but for Americans, don't say that children can't answer it, even adults can't. Most Americans have low IQ in arithmetic, so they will look stupid when they buy things in daily life. This problem only requires the least common multiple of the perimeter of a big circle and a small circle. The circumference of a small circle = 2π, the circumference of a big circle =3 times the diameter of a small circle multiplied by π, and the result comes out. The answer is 3.