Added missing comma in carArray arguments
Changed quotes to apostrophes, adding escapes where needed

Changed all of the quotes in the line to apostrophes for consistency and
added an escaped (\) the apostrophe in the first string.
Changed all quotes in script.js to apostrophes

Changed all quotes in script.js to apostrophes for consistency. Escapes
have been added where needed.
Removed overriding startTimer function

Removed the startTimer function, as requested by the comments in the
code (randomBackgroundColour was undefined anyway). This startTimer was
also overriding another in script.js, so the body onload call to
startTimer has been left in (comment says that "later I want to add some
more automated stuff")
Fixed missing images and naming

Fixed the image links so that none are missing. Updated the naming used
for each of the images to make it more consistent.
Simplified condition to greater than or equal to

Simplified the condition to a greater than or equal to comparison ( >= )
Updated purchase rate and corrected spelling

Updated the purchase rate to 10% as requested in the code. Corrected the
spelling, from pruchaseRate to purchaseRate.
Fixed age option values

Fixed the age option values so that they are integers instead of
strings, which allows the calculations to work correctly.
Fixed span to use id instead of class

Fixed span to use id instead of class so that the javascript can select
it using document.getElementById()
Update loan interest rate

Updated the interest rate so that it is 50% instead of 5000%