Answer:
60 m/h
Step-by-step explanation:
1. the basic formula is Time=Distance/Speed;
2. total time is time1 (for 84 miles)+time2 (for 130 miles)=time (4 hours);
3. the time for the first 84 mi. is time1=84/S;
4. the time for the rest 130 mi. is time2=130/(S-10);
5. using all the data of items 2-4, it is possible to substitute into the basic formula and resolve an equation:
![\frac{84}{s} +\frac{130}{s-10} =4; \ => \ 2s^2-127s+420=0;](https://tex.z-dn.net/?f=%5Cfrac%7B84%7D%7Bs%7D%20%2B%5Cfrac%7B130%7D%7Bs-10%7D%20%3D4%3B%20%5C%20%3D%3E%20%5C%202s%5E2-127s%2B420%3D0%3B)
![\left[\begin{array}{ccc}s=60\\s=3.5\end{array}](https://tex.z-dn.net/?f=%5Cleft%5B%5Cbegin%7Barray%7D%7Bccc%7Ds%3D60%5C%5Cs%3D3.5%5Cend%7Barray%7D)
note, that the value of speed has one condition: s>10. It means, the only value is true: 60 miles per hour.