GMAT Strategy

I am trying to determine how the picking numbers strategy works. Why does it work in certain situations and not others. For instance:

In this problem...
At a loading dock, each worker on the night crew loaded (3/4) as many boxes as each worker on the day crew. If the night crew has 4/5 as many workers as the day crew, what fraction of all the boxes loaded by the two crews did the day crew load?

Answer choices:
(A) 1/2
(B) 2/5
(C) 3/5
(D) 4/5
(E) 5/8

It was solved by niraj_a by picking numbers.


: Did this strategy work only because a fraction/percentage was asked for
: would it have worked if a value for the # of boxes loaded by one of the crews was asked for

niraj_a wrote:E

I solved this by picking numbers.

Say the Day Boxes /worker = 20 boxes
then Night Boxes / worker = 20 * 3/4 = 15 boxes

Now, say the Day workers = 30 workers
then the Night workers = 30 * 4/5 = 24 workers

Now we find how many boxes loaded in total by all workers -

Day Crew = 20 * 30 = 600
Night Crew = 15 * 24 = 960

So, to find the fraction of boxes loaded by the day crew -

600 / 960 = 5/8 = E.