Question 1036444
Most likely you meant 1/4 of the distance and 3/8 of the distance

1/4 of d + 12
3/8 of d + 9

we add 1/4 and 3/8 of the distance and we get 5/8 of the distance. If he biked 5/8 of the distance and also biked 12 additional and 9 additional miles to cover the rest of the distance this means that the rest of the distance which is 8/8 - 5/8 = 3/8 is equivalent to 9+12 = 21

3/8 * d = 21. d=21*8 divided by 3 = 7*8 = 56