TargetMBA007
I get the correct answer, when I find the speed of 6 machines by doing 6/27*4 = 1/18, hence 18 hours, and then minus 9 hours.
However, if I directly do:
6/(27*4)-(1/27), it produces = 1/54, or 54 hours. I wonder, what is fundamentally the error with the second approach?
KarishmaBRecall that we can easily solve these questions using the concept of Joint variation:
4.................27
6................. ??
T = 27 * (4/6)
because time taken will reduce when number of machines increases
T = 18 i.e. time taken will be 9 hrs less
In your second approach, you are calculating 'Rate of 6 machines - Rate of 4 machines' which gives you 'Rate of 2 machines' since rates are additive.
(you know Rate of 4 machines + Rate of 2 machines = Rate of 6 machines)
So 1/54 is the rate of 2 machines and 54 hrs is the time taken when only 2 machines are working. What we are actually asked is the simple subtraction 'time taken by 4 machines - time taken by 6 machines'