A computer can execute 36 instructions per microsecond.
How many instructions can it execute in 4 min?
4 min x (60 sec/1 min) x (1,000,000 microseconds/sec) x (36 instructions/microsecond) = ?? instructions
Note how the units we want to disappear (min, sec, and microseconds) do just that by canceling and the unit we want to keep (instructions)is the only one left.