For this problem, you need to write two linear equations that represent the cost for each rental company. The flat rate corresponds to the y-intercept while the cost per mile is equal to the slope. So you will use the slope-intercept form to write the equations. The slope-intercept form of a linear equation is y=mx+b where m is equal to the slope and b is equal to the y-intercept. The equations follow.
Mr. Wheels- y=.10x+90
Grand auto- y=.15x+75
Now, plug 100 into each equation to calculate the cost of traveling 100 miles.
Mr. Wheels- y=.10(100)+ 90=10+90=100
Grand auto- y=.15(100)+75+15+75=90
Therefore, Grand auto is the best choice if you are traveling 100 miles as it offers the lower price.
To calculate where the prices are the same, you need to solve the system of equations (simultaneous equations). You can use substitution or elimination. I will use substitution and substitute the y of the Grand auto equation into the y of the Mr. Wheels equation and solve for x.
.15x+75=.10x+90
.05x=15
x=300
So at 300 miles, the cost will be the same for each company. The cost will be $120 for both companies.