Think ...does the Broad street divides the city equally.i.e 50% of the east side and 50% of the west side? as far as the population is concerned?
you don't know and I believe it doesnt..therefore how can you add the two percentages for the whole city?...
Suppose the east side has 10000 sitizens and the west has 20000 citizens then the total population of the city is 30000 and the 50% is 15000 .
now get the 30% 0f 10000 they are 3000 get the 20% of the 20000 they are 4000 add them together....what you see? 7000 even the half of the 15000
therefore NO YOU CANNOT ADD THE TWO PERCENTAGES....