See how long it takes you to figure this one out: if something falls by 50 per cent, then rises by 100 per cent, where is it? Answer: just back where it started.
If you had to think about it you need to be careful what conclusions you draw from this week's national accounts showing the economy - real gross domestic product - contracted by 1.2 per cent in the March quarter.
Thanks to economists' obsession with growth, we focus almost exclusively on the percentage change in GDP and its components from one quarter to the next, but if you don't have a good feel for how percentage changes work you risk bamboozling yourself.
(Speaking of which, remember that, though the percentage increase needed to get you back to par is always bigger than the original fall, the smaller that fall the less spectacular the subsequent rebound.)
Now try this reaction to the national accounts from Joe Hockey: ''If the mining boom has a cough the Australian economy can suffer pneumonia. The economy is increasingly reliant on the mining boom.''
It's a snappy soundbite for the telly, but it's nonsense. Indeed, it's roughly the opposite of what the national accounts are telling us.
For a start, the problem during the March quarter wasn't the mining boom, it was the weather. Is our economy heavily reliant on the weather? Our farmers are, but the rest of the economy isn't (well, not until we're finally screwed by climate change).
For another thing, what happened last quarter wasn't a cough that shows we've got pneumonia, it was a hiccup that isn't worth worrying about. Remember, 98.8 per cent of the economy was still there in the March quarter.
All that happened was that flooding and cyclones temporarily disrupted our production of coal, iron ore, agriculture and tourism. The disruption to mining in particular led to a decline of 27 per cent in the volume of coal exports during the quarter, causing the volume of all exports to fall by 8.7 per cent.
But today, two months after the end of the March quarter, we know the bad weather has stopped, most mines are working again, farmers have replanted and the rebuilding of houses, roads and other infrastructure has begun. Export volumes recovered in the month of March and further in April.
The natural disasters are estimated to have subtracted 1.7 percentage points from real GDP growth during the quarter. But Wayne Swan is expecting a rebound of about 1 percentage point in the present quarter and a further rebound in the September quarter. This is why Hockey's pneumonia is no more than a hiccup.
The rebound will come for three reasons: production will return to normal; some firms will work overtime to catch up on lost production and there will be much rebuilding and purchasing of new equipment.
Note that, thanks to our obsession with rates of quarterly change, part of the rebound is simply arithmetic. The government estimates the various natural disasters subtracted $6.2 billion from the real value of production in the March quarter, but will subtract only $3.1 billion in the June quarter. If so, this reduction in a negative represents a positive contribution to the growth in real GDP in the June quarter.
The point is, when the economy contracts in a quarter, you have to investigate the causes before you decide the economy has pneumonia and needs to be hospitalised. In this case, the causes are transitory - and self-correcting - rather than lasting.
Another clue is that the economy suffered a weather-caused shock to its supply side (production of goods and services) rather than weakness in its demand side (spending on goods and services).
A weakness in demand is more likely to be deeper-seated and longer-lasting, requiring the economy's demand managers - the government and the Reserve Bank - to adjust the settings of the instruments they use to influence the strength of demand: respectively, fiscal policy (the budget) and monetary policy (interest rates).
What makes Hockey's talk of pneumonia so opposite to the truth is, when you look past the temporary supply problem you see demand growth is quite healthy. Consumer spending grew by 0.6 per cent in the March quarter (and by 3.4 per cent over the year to March), with government spending on consumption items growing by 1.4 per cent (4.6 per cent for the year).
Turning to investment spending, spending on new or altered housing grew by 4.6 per cent (annual, 6.6 per cent) and business investment in new equipment and structures grew by 2.9 per cent (annual, 4.5 per cent).
That leaves public sector investment spending, which fell by 0.7 per cent (annual, minus 6 per cent) as the fiscal stimulus continued to be withdrawn. (Overall, the withdrawal of stimulus trimmed 0.4 percentage points from GDP growth during the quarter.)
Adding this up, ''domestic final demand'' grew by a whopping 1.3 per cent during the quarter and by 3.3 per cent over the year. Allowing for a fall in the level of business inventories (much of it probably caused by the natural disasters), ''gross national expenditure'' - which is domestic demand proper - grew by a healthy 0.8 per cent during the quarter and by 3.1 per cent over the year.
This healthy growth ain't surprising since total employment grew by almost 50,000 during the quarter.
As the secretary to the Treasury, Martin Parkinson, pointed out this week, though the mining sector gets most of the headlines, it accounts for only about 8 per cent of GDP (and an even smaller proportion of total employment). So that leaves 92 per cent of the economy that's not mining but is doing fine.
It's true that, of late, much of the growth in the economy has been coming, directly and indirectly, from mining. But it's rare for all parts of the economy to be growing at the same rate, so it's common for one sector - often it's been housing - to account for much of the growth in a particular period. That doesn't mean we'd catch pneumonia were that sector to falter.
Consider this: if the sky-high prices we're getting for coal and iron ore were to suddenly collapse, that would be a blow, but it would also bring about changes that encouraged other sectors to grow faster: the high exchange rate would fall, the Reserve Bank would cut interest rates and the budget wouldn't be as contractionary, taking longer to return to surplus.
