The annual average rainfall in a tropical city is $1000$ mm. On a particular rainy day ($24$-hour period), the cumulative rainfall experienced by the city is shown in the graph. Over the $24$-hour period, $50$% of the rainfall falling on a rooftop, which had an obstruction-free area of $50$ $m^2$, was harvested into a tank. What is the total volume of water collected in the tank litters?
1. $25,000$
2. $18, 750$
3. $7,500$
4. $3.125$