I went through the following question:
If 5 spiders can catch five flies in five minutes. How many flies can hundred spiders catch in 100 minutes?
The answer is calculated by the following situation:
No. of spider-minutes(same as man-hours term) = $5 *\ 5 $ for $5$ flies i.e. $25$ spider-minutes for $5$ flies.
No. of spider minutes given $ = 100 *\ 100 $
Therefore no. of flies $ = \frac {100 *\ 100}{25} *\ 5 = 2000$
I think the last step means to say
No. of flies $ = \frac {Given \ spider \ minutes}{original \ spider \ minutes} *\ 5$
But what is co-relating these 2 situations?
Best Answer
$5$ spiders can catch $5$ flies in $5$ minutes: $\frac{5\times 5}{5\times 5}\times 5$
$100$ spiders can catch $100$ flies in $5$ minutes: $\frac{100\times 5}{5\times 5}\times 5$
$100$ spiders can catch $20 \times 100 =2000$ flies in $20 \times 5 = 100$ minutes: $\frac{100\times 100}{5\times 5}\times 5$
or
It takes $25$ spider-minutes to catch $5$ flies. You have $100 \times 100$ spider-minutes, so can catch $\frac{100\times 100}{25}\times 5$ flies.