Problem: the average age of a group of 10 students is 20 years . if 2 more students join the group , the average age increases by 1 year . the average age of the new students is
Rationale: "total age of 10 students = 10 * 20 = 200 and the total age of 16 students = 21 * 15 = 315 . let the average age of 2 new students be x . therefore total age of the new students = 2 x . so total age of 12 student = 200 + 5 x = 315 , x = 23 hence , the correct answer is a ."
annotated_formula: divide(subtract(multiply(add(20, 1), add(20, 1)), multiply(20, 10)), 2)