about the same as the previous posting
a = gallons used up by the 20 mpg car
b = gallons used up by the 30 mpg car
we know on that week they both consumed 65 gallons altogether, therefore a + b = 65.
we also know that they did a combined mileage of 1550. Now, in 1 gallon the 20mpg car does 20(1) miles, in 2 it does 20(2) miles, in 3 gallons it does 20(3) and in "a" it does 20(a) or 20a miles.
likewise, the 30mpg car does 30(b) in b gallons, or 30b miles.
Therefore, 20a + 30b = 1550.
[tex]\bf \begin{cases}
a+b=65\implies \boxed{b}=65-a\\
20a+30b=1550\\
----------\\
20a+30\left(\boxed{65-a} \right)=1550
\end{cases}
\\\\\\
20a-30a+1950=1550\implies 400=10a\implies \cfrac{400}{10}=a\\\\\\ \stackrel{\textit{gallons}}{40}=a[/tex]
how many gallons is it consumed by the 30mpg car? well b = 65 - a.
