EducationAisle wrote:
Hi happy19, this sentence bears close resemblance to the following official question:
In 1981 children in the United States spent slightly less than an average of two and a half hours a week doing household chores; by 1997 that figure had grown to nearly six hours a week.
I will copy and paste a very similar example and corresponding explanation from our book Sentence Correction Nirvana that will illustrate the reasoning behind this.
In 2007, a typical web user spent less than 4 hours a month on Facebook; by 2011, that figure had gone up to 8 hours.
There are actually 3 events that this sentence talks about:
(a) The time that a typical web user spent on Facebook in 2007
(b) 4 years passing since then (between 2007 and 2011)
(c) But, before those 4 years passed (within those years, as indicated by the words by 2011), that figure (the time that a typical web user spent on Facebook) went up to 8 hours.
However, note that (c) happened before (b) and hence, (c) should be in past perfect (in this case: had gone up). It should also be noted that in 2007 is a specific time, and so, a typical web user spent less than 4 hours a month on Facebook is correctly expressed in simple past (remember that simple past is used to express the situation that an event started and finished at a specific time in the past); on the other hand, by 2011 is an unspecified time because this could refer to any time in or before 2011), and so, a perfect tense (past perfect in this case) would correctly depict this situation.
I am hoping you can now easily draw parallels with the specific sentence in your post, and also the official sentence I have stated towards the beginning of my post here.
If you (or someone else) is interested, please PM me your mail-id and I can send you the section on Past-Perfect, from Sentence Correction Nirvana.
Thanks EducationAisle it clears my doubt. i found your analogy really helpful. I will try implementing it on more such situations and understand better.