00:01
Okay, so in this question, we are talking about how the population, the urban and rural population is changing with respect to time.
00:08
So u of t is the urban population and r of t is the rural population.
00:14
Okay, and we're told that currently the population is 25 % urban and 75 % rural.
00:21
So that means that if there were 100 people, 25 would be urban and 75 would be rural.
00:28
Now we're asked to give our initial conditions as decimals.
00:32
So we're actually going to give our initial conditions as a proportion of the population.
00:37
So u of zero would be 0 .25 because that's 25 % and r of zero would be 0 .75 since that's 75%.
00:49
Now what that means is that u and r are representing the percentage of the total population that's urban or rural rather than the actual number.
01:02
Okay, and now we want to select the correct system of differential equations modelling the rates of change in u of t and r of t.
01:09
Okay, so if we read our first bullet point, that tells us that each year 7 % of the urban population migrates to the rural countryside, while the other 93 % stays in the urban city.
01:22
Okay, so let's talk about how u is changing with respect to t...