First Part
Time walking = w
Time riding = r
time walking + time riding = 7 hours (total hours)
w + r =7
Subtract r from both sides and you get
w=7-r
Second part
Time walking at 4 MPH + time riding at 42 MPH = 199 miles (total miles)
4w + 42r = 199
Third part
subsitute (7-r) for w in second part
4(7-r) + 42 r =199
distribute the 4
28-4r + 42 r = 199
Combine like terms
28 +38r = 199
Subtract 28 from both sides
38r = 171
divide both sides by 38
r=4.5
He rode for 4.5 hours
He walked for 2,5 hours
4.5 +2.5 = 7 (total hours)