A rental car company charges a fixed fee of $30 plus $0.05 per mile. Let C represent the total cost of renting a car and driving it M miles

We know that renting the car will cost $30, regardless of whether you drive it any where or not

So far

C = $30

On the end of this equation, we want to be able to add another value, that can vary depending on the quantity of M (number of miles)

The second value is $0.05 multiplied by how many miles are travelled

Also expressed as $0.05M

Finally, just fix the two parts together

C = $30 + $0.05M