Problem: there are 9 fictions and 6 non - fictions . how many cases are there such that 2 fictions and 2 non - fictions are selected from them ?
Rationale: "number of ways of selecting 2 fiction books = 9 c 2 number of ways of selecting 2 non fiction books = 6 c 2 9 c 2 * 6 c 2 = 36 * 15 = 540 answer : c"
annotated_formula: divide(multiply(multiply(9, const_4), multiply(6, 9)), power(factorial(2), 2))