You are given a math word problem and you are supposed to make use of mathematical concepts like probability, combinatorics, linear equations, sequences/series, or simple operators like addition, subtraction, division, or multiplication to answer the given question. Choose the best option from the 5 given options and report that option.

[EX Q]: Question: The average marks of a class of 35 students is 45 and that of another class of 55 students is 65. Find the average marks of all the students?

 Option A: 52.6

 Option B: 52.4

 Option C: 52.1

 Option D: 57

 Option E: 52.9
[EX A]: Option D

[EX Q]: Question: A train covers a distance of 12 km in 10 min. If it takes 10 sec to pass a telegraph post, then the length of the train is?

 Option A: 100

 Option B: 110

 Option C: 200

 Option D: 130

 Option E: 140
[EX A]: Option C

[EX Q]: Question: Three houses are available in a locality. Three persons apply for the houses. Each applies for one house without consulting others. The probability that all the three apply for the same house is

 Option A: 1/7

 Option B: 1/9

 Option C: 1/6

 Option D: 1/4

 Option E: 1/2
[EX A]:
Option B