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: Two pipes A and B can fill a tank in 15 min and 20 min respectively. Both the pipes are opened together but after 4 min, pipe A is turned off. What is the total time required to fill the tank?

 Option A: 14 min 19 sec

 Option B: 14 min 42 sec

 Option C: 14 min 20 sec

 Option D: 14 min 40 sec

 Option E: 14 min 10 sec
[EX A]: Option D

[EX Q]: Question: In what time will a train 200 m long cross an electric pole, it its speed be 144 km/hr?

 Option A: 2 sec

 Option B: 2.7 sec

 Option C: 3.5 sec

 Option D: 5 sec

 Option E: 9.5 sec
[EX A]: Option D

[EX Q]: Question: At a certain restaurant, the average (arithmetic mean) number of customers served for the past x days was 80. If the restaurant serves 120 customers today, raising the average to 90 customers per day, what is the value of x?

 Option A: 2

 Option B: 3

 Option C: 9

 Option D: 15

 Option E: 30
[EX A]:
Option B