Question 793164
Let y be the rate in second half and x the total distance

t=d/r

time first half + time second half = time for total journey

(x/2)/(15)+ (x/2) /y = x/20

x/30 -x/20 = - x/2y

(2x-3x)/60 = -x/2y

-x/60 = -x/2y

2y=60

y=30 km/h