Question

the family stopped to rest after covering 3/5 of the distance. they still had 750 miles to go. how far did they travel?

Answer 1

750/2=375  5*375=1,875 (total distance) 3*375=1,125 (traveled distance).  Your answer should be 1,125 miles. 

Answer 2

The family still had 1 - 3/5 = 2/5 of the whole distance to drive (750 miles to go).

x ... the whole distance
2/5 * x = 750    /*5/2
x = 750 * 5 / 2
x = 1875 miles

If you would like to know how far did they travel so far, you can calculate this using the following step:
3/5 of x = 3/5 * 1875 miles = 1125 miles

Result: The family stopped to rest after covering 1125 miles.