If it takes 4 days to complete 5/7 of the trip, it took 4/5 of a day to complete 1/7 of the trip. It will take 7 times as long to complete the trip because 7 sevenths make one whole, so it would take 7×4/5=28/5 days to do the whole trip. But 4 days have already been taken so far so we need to subtract 4 days from 28/5 days. 4 days is 4×5/5=20/5 days. 28/5-20/5=8/5 days to go before the trip is completed.
Another way is to say that if 5/7 of the trip is completed then 2/7 of the trip is left to complete. We know that 1/7 of the trip takes 4/5 day so 2/7 takes twice as long: 8/5 days to go to complete the trip.
[The whole trip is 1250 miles. It takes 28/5 days to travel 1250 miles, so it takes 1/5 day to travel 1250/28 miles and in one day 5 times this distance is completed: 5×1250/28=6250/28=3125/14=223³⁄₁₄ miles a day.
To complete the trip we need 8/5 days which is a distance of 8/5×3125/14=8×625/14=4×625/7=2500/7=357¹⁄₇ miles.]