Lack of math skills come up all the time. For years I have been chuckling
over "1.9% financing or $3000 cash back" ads. If one elects to get the
financing and forgo the $3000 cash back, then the actual interest rate is
much, much higher.
On a $20,000 loan, not getting the cash back raises the effective interest
rate to about 13%, higher than most banks. This assumes a simple loan; many
dealerships actually structure the plan even more obnoxiously using weird
variations on the APR formula.