Let
y ----> total cost in dollars
x -----> number of miles
we have that
Option B
y=50
Option A
Find out the slope
take two points from the graph
(0,20) and (150,50)
m=(50-20)/(150-0)
m=30/150
m=1/5=$0.20 per mile
therefore
y=0.20x+20
Part A
For x=75 miles
Option A -----> y=0.20(75)+20=$35
Option B ------> y=$50
therefore
Option A costs less
Find out the difference
$50-$35=$15
Cost $15 less than the other option
Part B
Looking at the graph
For x=150 miles
Both options cost the same
If David drives more than this amount option B costs less