speed of water=2.52km/hr=2520m/hr
length = 2520m/hr
let the radius of pipe = r
and radius of tank =40 cm=4/10m
length of tank in half hour = 3.15m
length of tank in an hour = 2*3.15m =6.3m
now,
volume of pipe = volume of tank
pie *r ^2 * 2520= Pie *4/10*4/10*6.3
r^2 =1/2500 m
r= 1/50 m
diameter = 2/50 m = 200/50 cm = 4 cm