EXPLANATION
Assuming that x represents the number of oil changes hours, the expression will be:
[tex]Fred\longrightarrow8+1x[/tex][tex]Gabriel\longrightarrow0\text{ + 3x}[/tex]If the friends are tied, and have the same number of oil changes, we can match both equations in order to get the number of hours:
[tex]8+x=3x[/tex]Subtracting -x to both sides:
[tex]8=3x-x[/tex]Subtracting like terms:
[tex]8=2x[/tex]Dividing both sides by 2:
[tex]\frac{8}{2}=x[/tex]Simplifying:
[tex]4=x[/tex]Plugging in x=4 into each expression:
[tex]Fred\longrightarrow8+4=12\text{ changes}[/tex][tex]\text{Gabriel }\longrightarrow\text{ 3}\cdot4\text{ }=\text{ 12 changes}[/tex]In conclusion, in 4 hours, both men will have completed 12 oil changes.