Question 317832
For each car, (miles driven)/(gallons consumed) = miles/gallon
Let {{{a}}} = average miles driven by 1st car
Let {{{b}}} = average miles driven by 2nd car
given:
(1) {{{a + b = 1200}}} miles
{{{a/30 + b/40 = 35}}}
Multiply both sides by {{{120}}}
(2) {{{4a + 3b = 4200}}}
Multiply both sides of (1) by {{{3}}} 
and subtract (1) from (2)
(2) {{{4a + 3b = 4200}}}
(1) {{{-3a - 3b = -3600}}}
{{{a = 600}}}
{{{b = 1200 - 600}}}
{{{b = 600}}}
The ave gas milage for {{{a}}} is {{{600/30 = 20}}} mi/gal
The ave gas milage for {{{b}}} is {{{600/40 = 15}}} mi/gal