A cone has radius 9 and height 12. A frustum of this cone has height 4. What is the lateral area of the frustum? The radii of the frustum is 9 and 6.
The slant height of the frustum is 5.​

there is only one way to arrange all the eight relative to each other because they are not distinct. we have

now there are nine places to place an (and not more than one in one place).   because we have four symbols, we need to choose four of these nine places to insert an . order does not matter because the are not distinct.

thus there are 1 · c(9,4) ways, or, if we calculate it,

thus there are 126 ways to form such "words."