There are many ways to do this such as solve the first two tier, then use the result to subtract
the third tier from. Another way is to add the two bottom tier together then subtract the result
from the top tier. Else you can do all three at one go.
My favorite technique is the second one because for a three-tier, I may need to borrow
2(tens) sometimes and adding the bottom two numbers together is the quickest way
to decide. In this case, a borrow becomes,
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