A
Speed of the rider = 40km/hr.
Distance covered in 1st hour = 40 km.
He increased his speed in every 1 hour by 4 km/hr.
Distance covered in every 1 hours will be, 44, 48, ,... upto 12 terms.(for 12 hours).
The above series is an A.P series;
Sum of first n terms = (n/2)(2a+(n-1)d)
Here, a = 40, d = 4 and n = 12.
Sum of first 12 terms = (12/2)(2(40)+(11)4) = 6(80 + 44)
= 6(124) = 744.
Hence, he covers 744 km in 12 hours.