What is a real-life situation that can be described using an arithmetic sequence with three missing terms?

Let's say, you have a bank account will a certain sum of money, and with a service of simple interest of 10% per annum.

Say, you have $100 000, so the interest is $10 000 per annum.
Say, you have to obtain $150 000. You can make use of arithmetic progression to estimate the amount of years you'll have to wait until you get your required sum.

However, there can be more than 3 missing terms here. If you do the calculation, you'll see it's for 5 years (5 missing terms).

Does that help? :)