the average age of a class of 19 students is 20 years . the average increased by 1 when the teacher ' s age also included . what is the age of the teacher ?
total age of all students = 19 ã — 20 total age of all students + age of the teacher = 20 ã — 21 age of the teacher = 20 ã — 21 â ˆ ’ 20 ã — 19 = 20 ( 21 â ˆ ’ 19 ) = 20 ã — 2 = 40 answer is c .
subtract(multiply(add(19, 1), add(20, 1)), multiply(19, 20))