Question 76
Sara has 10 blocks numbered $1$ to $10$. She wants to stack all the blocks into a tower. A block can only be put on top of a block with a higher number, or on top of a block with a number that is exactly one lower. An example of such a tower is, from top to bottom: $2$, $1$, $5$, $4$, $3$, $6$, $7$, $9$, $8$, $10$.
How many different towers are possible?
How many different towers are possible?