Answer:
18. compound interest
19. simple interest
20. simple interest
Explanation:
For these problems, the initial balance is irrelevant. All that matters is the multiplier of that balance. For simple interest at rate r for t years, the multiplier is ...
simple interest multiplier = (1 +rt)
For interest compounded annually, the multiplier of the initial balance is ...
compound interest multiplier = (1 +r)^t
A spreadsheet can do the computations for you.
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As an example of the computations involved, consider problem 19:
simple interest multiplier = 1 + 0.13·6 = 1.78
compound interest multiplier = 1.10^6 = 1.771561
The latter is less than the former, so the simple interest account will have the (slightly) greater balance at the end of 6 years.