Question 187393
For the 30 mi/hr car:
(1) {{{d[1] = r[1]*t[1]}}}
For the 46 mi/hr car:
(2) {{{d[2] = r[2]*t[2]}}}
given:
{{{t[1] = t[2]}}}
{{{r[1] = 30}}}
{{{r[2] = 46}}}
------------------
{{{d[2] = d[1] + 72}}}
The faster car goes 72 mi further than the slower car
I can rewrite (1) and (2)
(1) {{{d[1] = 30t[1]}}}
(2) {{{d[1] + 72 = 46t[1]}}}
(2) {{{d[1] = 46t[1] - 72}}}
Note that {{{t[1] = t[2]}}} since they travel different
distances but in the same amount of time
Since both left sides are {{{d[1]}}}, I can set the 
right sides equal to eachother
{{{30t[1] = 46t[1] - 72}}}
{{{16t[1] = 72}}}
{{{t[1] = 4.5}}} hr
and 
{{{t[2] = 4.5}}} hr also
In 4.5 hrs they will be 72 mi apart
Now check the answer:
(1) {{{d[1] = 30t[1]}}}
(1) {{{d[1] = 30*4.5}}}
(1) {{{d[1] = 135}}} mi
and
(2) {{{d[1] + 72 = 46t[1]}}}
(2) {{{135 + 72 = 46*4.5}}}
(2) {{{207 = 207}}}
OK