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:
[tex]\frac{84}{s} +\frac{130}{s-10} =4; \ => \ 2s^2-127s+420=0;[/tex]
[tex]\left[\begin{array}{ccc}s=60\\s=3.5\end{array}[/tex]
note, that the value of speed has one condition: s>10. It means, the only value is true: 60 miles per hour.
