Part a:
Interest for 6 months = 2.4%
Interest for 9 months = 2.9%
2.4/100 = 0.024
0.024 * 900 = 21.6
2.9/100 = 0.029
0.029 * 900 = 26.1
926.1 - 921.6 = 4.50
$4.50 more interest is owed.
Part b:
No, the rate does not increase at a constant rate.
2.9 - 2.4 = 0.5. There is a 0.5 increase from 6 months to 9 months, which is 3 months.
3.0 - 2.9 = 0.1. There is a 0.1 increase from 9 months to 12 months, which is also 3 months.