Better: The *amount* added in year $ n $ is $ I_n $, where $ I_1 = 100 $, $ I_2 = 0.9 \times 100 = 90 $, $ I_3 = 0.9 \times 90 = 81 $, $ I_4 = 72.9 $, $ I_5 = 65.61 $