A friend purchased a new sound system that was selling for $4200: He agreed to make a down payment of $1000 and to make 24 monthly payments of $160; beginning one month from the time of purchase. What...

A friend purchased a new sound system that was selling for $4200: He agreed to make a down payment
of $1000 and to make 24 monthly payments of $160; beginning one month from the time of purchase.
What is the effective interest rate being paid?

(pls solve showing every step with the mathematical formulas, do not use excel ect.)