Henry paid a total of $360 for the dishwasher: $20 + (10 x $34) = $20 + $340= $360
Therefore he paid $40 extra which we place over the original amount of the dishwasher:[tex] \frac{40}{320} = \frac{4}{32} = \frac{1}{8} [/tex]
Dividing 1 by 8 we get .125 which, when changed to a percentage becomes 12.5%
Henry paid a yearly interest rate of 12.5%