For a catchment area of 180 km2, a unit hydrograph, plotted for a one-hour duration, takes a triangular shape with its peal discharge of 50 m3/sec at 10 hours and a time base of 20 hours. The phi index is 0.4 cm per hour and the base flow is 10 m3/sec. If there is a rainfall of 3.2 cm in hour, the ordinate of flood hydrograph at the 15th hour is ______
1
80 m3/sec
2
70 m3/sec
3
62 m3/sec
4
52 m3/sec