Problem Solving

If in a certain population, people are born at an average rate of 9 people every 2 seconds and people die at an average rate of 3 people every 2 seconds, by approximately how many people does the population increase in 1 day?

17, 280
25,920
129,600
259,200
388,800

I got the correct answer, but it took me too long. I'm sure there is something I'm not seeing, can anyone tell me if there is quick way to solve a problem like this one?

thanks.

Ans should be 'D'.

From q every 2 second the gain of people is 9-3=6

I day=24*3600 seconds
So every 2 seconds the gain is 6 people
for 24*3600 seconds gain =(24*3600)/2 * 6 =259200


Amit