2, 1, 6, 0, 5, 9 অংকগুলাে দ্বারা ছয় অংক বিশিষ্ট কতগুলাে সংখ্যা গঠন করা যেতে পারে ?

Let $S$ be the set of $6$ digits numbers $a_{1},a_{2},a_{3},a_{4},a_{5},a_{6}$ (all digits distinct) where $a_{1}>a_{2}>a_{3}<a_{4}>a_{5}<a_{6}$. Then $n(S)$ is equal to

Which of the following is equal to $\dfrac{1.3.5....(2n-1)}{2.4.6....(2n)}$?

  $\frac{1}{1 !} ÷ \frac{1}{0 !}$ =কত? 

The letters of word $OUGHT$ are written in all possible orders and these words are written out as in a dictionary. Find the rank of the word $TOUGH$ in this dictionary.