Look again at the series Z 7 @ 4 P % C 9 5 T E # 2 S 8 U & 6 A 3 W !. How many elements lie between the letter T and the letter U?

T stands at place 10 and U at place 16. The elements strictly between them run from place 11 to place 15 — E, #, 2, S and 8 — which is 5 in all. Answering 6 comes of counting one of the two named letters as lying between them, and 7 of counting both; 4 comes of dropping one element from the run.