Problem Solving

Could someone explain the below?




Six machines, each working at the same constant rate, together can complete a certain job in 12 days. How many additional machines, each working at the same constant rate, will be needed to complete the job in 8 days?

A) 2
B) 3
C) 4
D) 6
E) 8

From Official GMATPrep Software

OA is B

Six machines, each working at the same constant rate, together can complete a certain job in 12 days. How many additional machines, each working at the same constant rate, will be needed to complete the job in 8 days?

2
3
4
6
8

Approach 1: Plug in a rate for EACH MACHINE

Let the rate for each machine = 1 unit per day.
Rate for 6 machines = 6 units per day.
In 12 days, the amount of work produced by 6 machines = r*t = 6*12 = 72 units.
To produce 72 units in 8 days, the required amount of work per day = w/t = 72/8 = 9 units per day.
To increase the rate from 6 units per day to 9 units per day, 3 more machines are needed.

The correct answer is B.

Approach 2: Inverse proportion method

The number of machines is INVERSELY PROPORTIONAL to the number of days:
(machines)(days) = (machines)(days).
As the number of machines INCREASES, the number of days must DECREASE, so that in each case the SAME AMOUNT OF WORK is produced.

Since 6 machines take 12 days, and the job is to be completed in 8 days, we get:
6 * 12 = m * 8
72 = 8m
m = 9.

Since 9 machines are required, the original number of machines -- 6 -- must increase by 3.

The correct answer is B.