Setting Up The Problem

We really have much concrete data to go on from this newspaper story, but we can still make some reasonable guesses about the quantities involved. We could probably look up a lot of the exact values for what follows in the literature, but let's not do that (it's tedious work, after all).

Firstly, let's estimate Sydney's motor vehicle fleet as consisting of one million vehicles. It could be half a million, or it could be one and a half million - it doesn't really matter for the purposes of this exercise. Our answer is probably only going to be correct to one significant figure anyway; what is important is that we will get approximately the right order of magnitude for our answer.

We will assume that roughly half the motor vehicles (so about 500 000) still use leaded fuel. This is probably slightly generous, but again, we are only after a rough answer.

The average distance travelled in a day by a motor vehicle will be taken to be 50 km.

So the total distance travelled by all of the cars that use leaded petrol will be about 25 000 000 km / day.

(** Note: At this point, you might think that we should write this as 25 x 10^6 in order to get the significant figures correct. However, as has explained earlier, we are really only after an "order of magnitude" estimate, since we have acknowledged that our estimates are extremely rough right from the beginning. So we will basically ignore significant figures throughout this exercise and then right at the end we must remember that our answer is probably only accurate to one significant figure.)