IN finding the interest we need to use the following formula:
Interest= Principal x Rate x Time or I= PRT
Substitute values: I= [$20 + ($10 x $34)] -320 =$40
P= $320
R=?
T=10 months/year
I=PRT
Since R is a missing term, we will solve for R using this formula: R=I/PT
R= [$20 + ($10 x $34)-320] / ($320 x 10 months)
T=10 months÷12 months=0.83
R= ($40)/ $320 X 0.83
R= 40/ 265.6
R=15.06024096