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.
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Question: Question: A cistern can be filled by a tap in 3 hours while it can be emptied by another tap in 9 hours. If both the taps are opened simultaneously, then after how much time will the cistern get filled?

 Option A: 2.9 hrs

 Option B: 4.5 hrs

 Option C: 2.9 hrs

 Option D: 7.2 hrs

 Option E: 8.6 hrs

Answer: Option B


Question: Question: If x, y, and z are positive integers such that x is a factor of y and y is a factor of z, which of the following is NOT necessarily an integer?

 Option A: (x + y

 Option B: (y + z

 Option C: yz/x

 Option D: (2y + 3z

 Option E: (xy + z

Answer: Option A


Question: Question: The number of ways in which one or more balls can be selected out of 10 white, 9 green and 7 blue balls is

 Option A: 892

 Option B: 881

 Option C: 891

 Option D: 879

 Option E: None of these

Answer:
Option D