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*
