Given $L$, $s$ and $d$, which are positive real numbers, is there always a pair of integers k and k', such that $kd\in [k'(L+s)-s, k'(L+s)]$. It is like there is a line which is painted red of length $L$ and painted blue of length $s$ next and so on. Suppose a man is walking with stepsize $d$. Whether the man will walk on the blue line at some time or not? We can suppose the blue line part is a closed set but the red line is an open set.