A triangular direct runoff hydrograph due to a storm has a time base of $90$ hours. The peak flow of $60\: m^3/s$ occurs at $20$ hours from the start of the storm. The area of catchment is $300\: km^2$. The rainfall excess of the storm (in cm), is
1. $2.00$
2. $3.24$
3. $5.40$
4. $6.48$