Answer:
2.735%
Explanation:
The amount due on principal P after t years at rate r is ...
A = P(1 +rt)
Filling in the given values, we can solve for r:
20,953.80 = 18000(1 +6r)
20953.80/18000 -1 = 6r = 0.1641
r = 0.1641/6 = 0.02735 = 2.735%
Joel's interest rate was 2.735%.