I believe the answer would be 19.61
If you divide the 18 mi by 4m to get how much mi there is in 1 m, you get 1m=4.5mi
After that u divide 4.5 by the remainder of 6.8 which is 2.8. (6.8-4)
Next you divied 4.5 by 2.8 and get 1.61
Add 18 and 1.61 to get your answer
Its possible i got this wrong but hopefully i helped