What is the value of
- A)
- B)
- C)
- D)
- E)
Answer
D
Key insight
Work from the inside out. Each layer is 3 plus a fraction: add, then flip the result upside down to get the fraction for the next layer up.
Solution
The fraction is built in layers. Only the innermost layer can be worked out right away, so start there and move outward.
- Innermost layer: .
- The layer above divides by this. Dividing by a fraction just turns it upside down: .
- Middle layer: .
- Turn it upside down again: .
- Outer layer: .
Quick check: the answer should be a little more than , and .
The answer is .
Why this works
Each layer of a stacked fraction like this is "add , then flip." You cannot finish an outer layer until you know the one under it, so always begin at the bottom. Keep every result as a fraction rather than a decimal, so each flip stays exact.
The trap
Flipping a fraction the wrong way, for example treating 1 divided by 10/3 as 10/3 instead of 3/10, or trying to start from the outside.
Concise solution
Work from the innermost level outward, simplifying one layer at a time and keeping every result as a single fraction.
Innermost layer: , whose reciprocal is .
Middle layer: , whose reciprocal is .
Outer layer: .
The answer is .
Review in the AoPS books
- Introduction to Number Theory, Chapter 11: Decimals and Fractions
- Prealgebra, Chapter 6: Decimals (§6.3, §6.4)
Techniques
Set up the equation/formula and compute; no special trick needed