Answer:
Without going to tables:
Suppose 1$ originally deposited
After 18 years using A (1 + i)^18 = 1.04^18 = 2.0258 $
Interest earned on the 1$ was 1.0258
How many $ deposited to earn $72
72 / 1.0258 = 70.18
So the original deposit was 70.188
Check:
70.188 * (1.04)^18 = 142.19 Collected at 18
If 70.18 was initially deposited then
142.19 - 70.18 = 72 is the interest earned
