Art or instinct or learned art??

0Hi,02br
02br
00The mixed use of past and past perfect seems to be OK as long as I can follow through with it but some cases like this is hard to follow through with the two main streams of tenses, especially the shifting back by the use of 'had reached'. Is this normal?02br
02br
00The rapid increase of the population01u00 worsened02u00 the economic situation. The population in August XXXX 01u00was 02u00abut 15 million, and it 01u00had reached02u00 17.3 million by September XXXX, 20 million in XXXX, and then 21 million in XXXX. 0-