Interest formula: A = P(1 + rt)
Solving for r (Rate): r = (1/t)(A/P - 1)
Rate = (1/2)(2500/1800 - 1) = (1/2)(25/18 - 1) = (1/2)(7/18) = 7/36 ≈ 0.194
Rate = 0.194% per annum
4/11 = 0.36 (repeat: 36)
5/11 = 0.45 (repeat: 45)
6/11 = 0.54 (repeat: 54)
7/11 = 0.63 (repeat: 63)
