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In the diagram, there are 26 levels, labelled A, B, C, . . . , Z. There is one dot on level A. Each of levels B, D, F, H, J, . . ., and Z contains twice as many dots as the level immediately above. Each of levels C, E, G, I, K, . . ., and Y contains the same number of dots as the level immediately above. How many dots does level Z contain?

  1. 2048
  2. 4096
  3. 8192
  4. 16384
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Since level C contains the same number of dots as level B and level D contains twice as many dots as level $\mathrm{C}$, then level $\mathrm{D}$ contains twice as many dots as level $\mathrm{B}$.

Similarly, level F contains twice as many dots as level D, level H contains twice as many dots as level $\mathrm{F}$, and so on.

Put another way, the number of dots doubles from level B to level D, from level D to level F, from level $\mathrm{F}$ to level $\mathrm{H}$, and so on.

Since there are 26 levels, then there are 24 levels after level B.

Thus, the number of dots doubles $24 \div 2=12$ times from level $\mathrm{B}$ to level $\mathrm{Z}$.

Therefore, the number of dots on level $\mathrm{Z}$ is $2 \times 2^{12}=2^{13}=8192$.
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