3 3/5% can be changed to 3.6
The multiplayer would be 1.036
So-
20000 x 1.036 = 20720
20720 x 1.036 = 21465.92
21465.92 x 1.036 = 22238.6931
22238.6931 x 1.036 = 23039.2861
23039.2861 x 1.036 = 23868.7004
The figure after is 23868.7
23868.7- 20000 = 3868.7
The interest earned is $3869.70
Hope this helps :)