Answer:
23 miles
Step-by-step explanation:
D rides x mph for 0.75 hr or 0.75x for d
F rides (x+6) mph for 0.5 hr for (x/2)+3 d
0.75x+0.5x+3=23 miles, so 1.25x=20 miles
x=16 mph for D, and he rides 12 miles in 3/4 hour
x+6=22 mph for F, and he rides 11 miles in 1/2 hour.