x = miles riding

y = miles walking

x + y = 72 total trip was 72 miles

x/15 + y/4 = 7 took 7 hours

x + y = 72 top equation stays same

4x + 15y = 420 multiplied bottom equation by 60 to clear fractions

-4x - 4y = -288 multiplied top equation by -4

4x + 15y = 420 bottom equation stays same

-----------------

11y = 132 added the equations

y = 12 divided by 11

x + y = 72

x + 12 = 72 substituted 12 for y

x = 60 subtracted 12

**60 miles riding
**

12 miles walking*McGenius*

**
**