In a test, the 10 students in group 1 get a mean score of 43%.
the 15 students in group 2 get a mean score of 57%.
hank says, " the mean score for all 25 students is 50%."
what mistake has hank made?


what is the correct mean score for all 25 students?

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."