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.

[Q]: Question: Working alone at its constant rate, machine K took 3 hours to produce 1/5 of the units produced last Friday. Then machine M started working and the two machines, working simultaneously at their respective constant rates, took 6 hours to produce the rest of the units produced last Friday. How many hours would it have taken machine M, working alone at its constant rate, to produce all of the units produced last Friday?

 Option A: 10

 Option B: 12

 Option C: 15

 Option D: 25

 Option E: 30
[A]: Option C


[Q]: Question: One water pump can fill half of a certain empty tank in 7 hours. Another pump can fill half of the same tank in 7.5 hours. Working together, how long will it take these two pumps to fill the entire tank?

 Option A: 7 7/13

 Option B: 1 5/8

 Option C: 3 1/4

 Option D: 7 7/29

 Option E: 3 1/2
[A]: Option D


[Q]: Question: If the number of 89n,870, where n represents the thousands digit, is a multiple of 9, then the value of n could be which of the following?

 Option A: 0

 Option B: 1

 Option C: 3

 Option D: 4

 Option E: 6
[A]:
Option D