Four parallel lines cut a transversal so that the three intercepts on it are equal to one another. A second transversal crosses the same four lines, and on it the distance from the first line to the fourth is 21 cm. What is the length of the middle intercept on that second transversal (in cm)?

If a set of parallel lines makes equal intercepts on one transversal, it makes equal intercepts on every transversal. The 21 cm is therefore cut into three equal pieces of 21 ÷ 3 = 7 cm each, so the middle intercept is 7 cm. The value 10.5 halves the whole; 5.25 divides by 4, counting the lines instead of the three gaps between them; 21 is the whole distance, not one intercept.