Let total distance travelled by horse rider be 2x.

Now, the time taken to cover first half distance is tAB=5x
Second half distance x between BC is covered in two parts, x=d1+d2
here d1 is the distance travelled with 10ms−1 and d2 is the distance travelled with 15ms−1 in equal time intervals 2t each.
Then, d1=210t,d2=215t
Second half distance x=2t(10+15)=225t
Now, the mean speed of rider is v=5x+252x2x=5+22×25=750ms−1
Thus, the value of x=50.