Here is a step-by-step explanation for your problem:
Step 1: Calculate the amount of the first deposit after one year
First deposit: $20,839
Interest earned on first deposit: (20,839 x 10.91%) = $2,269.82
Total amount after one year: 20,839 + 2,269.82 = $23,108.82
Step 2: Calculate the amount of the second deposit after one year
Second deposit: $22,872
Interest earned on second deposit: (22,872 x 10.91%) = $2,511.33
Total amount after one year: 22,872 + 2,511.33 = $25,383.33
Step 3: Calculate the amount of the final deposit after one year
Final deposit: $20,217
Interest earned on final deposit: (20,217 x 10.91%) = $2,214.93
Total amount after one year: 20,217 + 2,214.93 = $22,432.93
Step 4: Calculate the total amount available after four years
Total amount available after four years = 23, 108.82 + 25,383.33 + 22,432.93 = $71,925.08