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1) You're able to use the fastest, safest way to arrive at the answer, such as my above method.
2) You can arrive at the answer through simplification of fractions, canceling our variables, etc.
It should NOT apply towards "estimating" the answer. Unless you have 10 seconds left to guess an answer, you should ALWAYS use the calculator if you cannot do the mental math, such as in this case (if anyone can do 26620/(1.1^3) in their head faster than a calculator, you're pretty good). Setting up that equation and getting an answer from the calculator shouldn't take more than 30 seconds.