Question 512387
(mileage with AC (mi/gal))*(gas used with AC(gal))+ (mileage without AC (mi/gal))*(gas used without AC(gal))=total miles traveled (miles)
Note how the units cancel out to leave you with miles
Assign x for one of the missing elements.  We'll say the gas Jennifer used when she was using AC= x.  Since she used 91 gallons total the expression that would tell you what she used without AC= 91-x. <br> 
Plug in what you know and solve for x
(21)*(x)+(16)*(91-x)=1751
21x+1456-16x=1751
5x+1456=1751
5x=295
x=59<br>
So she used 59 gallons when she had AC.  To find how much she used without 
91-59=32<br>
You can find how many miles she drove during each by multiplying the mileage with the corresponding number of gallons. So...
21*59=1239
16*32=512