If the length varies inversely with the width, then the product of the length and width is constant. If the length of one rectangle is 18 and the width is 10, then the constant of proportionality is 18 x 10 = 180. To find the width of a rectangle whose length is 30, we can set up the equation:
length x width = 180
30 x width = 180
width = 6
Therefore, the width of the rectangle whose length is 30 is 6.