We will have the following:
His speed on the return (r) trio was 4mph faster than his speed in the first trip (o), so:
[tex]r=o+4[/tex]Then, we have that speed times distance equals time, and distance is equal to time over speed. Thus:
[tex]6h(\frac{x}{\text{hours}})=4h(\frac{x+4}{\text{hours}})[/tex]Then, we have:
[tex]6x=4x+16\Rightarrow2x=16[/tex][tex]\Rightarrow x=8[/tex]So, the initial speed was of 8mph.
Then we deternine the distance:
[tex]d=(8mi/h)(6h)\Rightarrow d=48mi[/tex]So, the two cities are 48 miles apart.