(a)[2]
In how many distinct ways can the $11$ letters of REQUIREMENT be arranged?
(b)[1]
In how many different ways can the $11$ letters of REQUIREMENT be arranged if the two Rs stay together and the three Es stay together?
(c)[3]
In how many different ways can the $11$ letters of REQUIREMENT be arranged if there are exactly three letters between the two Rs?
(d)[4]
Five of the 11 letters in REQUIREMENT are chosen. How many possible selections include at least two Es and at least one R?