At what speed would mack have to drive to complete 550 miles in 12 hours if he takes a 20-minute rest break after 3 hours of driving.

12/3 is 4 breaks 12x60 is 720mins at most with 4x20= 80 = 720-80=640/ 60 means we have 10.67 hours left 550/10.67= 51.55 mph

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."