How would you solve this aptitude test for army?

Question

In one hour, a machine can dig a hole with diameter 10 cm through a 1.75 m depth of consistently hard rock. The machine is required to dig to a total depth of 112 m. If the machine is used for a total of 7 hours per day, how many days will it take for the machine to complete the task?

               

(A) 9

               

(B) 10

               

(C) 27

               

(D) 28

Answer is B according to the book but how did they get the answer? I keep getting A

Answer 1

I would call this a poorly worded question. The answer of course is 9 and 1/7 days, which is not one of the choices. But it is closer to 9 than 10. The only justification for calling B (10) the 'right' answer is that, if you're asked "how long will this take?", and you tell the project manager "9 days", then on the 10th day your project will be 'late'. So, in this case, it may be better to round up to 10.