Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.

[Q]: Problem: in a group of 100 people, 90 have an age of more 30 years, and the others have an age of less than 20 years. if a person is selected at random from this group, what is the probability the person ' s age is less than 20 ?
Options: a. 0.1, b. 0.55, c. 0.65, d. 0.75, e. 0.85
[A]: a


[Q]: Problem: what is the probability of getting a queen or spade from a set of cards ?
Options: a. 3 / 13, b. 4 / 13, c. 5 / 13, d. 6 / 13, e. 7 / 13
[A]: b


[Q]: Problem: the average age of 36 students in a group is 18 years. when teacher ' s age is included to it, the average increases by one. what is the teacher ' s age in years ?
Options: a. 35 years, b. 55 years, c. 51 years, d. 54 years, e. none of these
[A]:
b