in how many ways can 21 books on english and 19 books on hindi be placed in a row on a shelf so that two books on hindi may not be together ?
"we must place all these books as under : xexex … . xex there are 21 books on english , 19 books on hindi 19 places out of 22 can be chosen 22 c 19 = 22 c 3 = 1540 ways ans : a"
add(add(choose(subtract(21, const_1), const_3), permutation(subtract(21, const_1), const_2)), choose(subtract(21, const_1), const_1))