Let's say I$\text{'}$m buying a truck, and I$\text{'}$m offered the two following payment options:

$1.$ $$50,000$ in cash today, or;

$2.$ $$20,000$ as a downpayment right now, then monthly payments of $$1,300$ for the next $3$ years $(36$ months, with the first payment due a month from today$)$

What would the effective monthly interest be if I were to choose the second payment option?

If you can, please use a method where you find the EMR by through your own math, as well as another method using the IRR function in Excel, showing all steps and calculations in both methods.

I$\text{'}$m trying to use the IRR method myself, but Excel keeps spitting out NUM#$,$ as shown in the screenshot.