Let's walk through this step by step.
Key Idea: every time we say "p percent of something," we are multiplying by p/100.We do this
5 times in a row:
1. p% of all galaxies have solar systems
2. p% of those have planets
3. p% of those have life-possible planets
4. p% of those have intelligent life
5. p% of those have sent a spaceship
Each step multiplies by p/
100. So after
5 steps, the fraction of all galaxies that have sent a spaceship is:
(p/100) × (p/100) × (p/100) × (p/100) × (p/100) = p^5 / 100^5 = p^5 / 10^10Now here's the crucial part that trips people up:
p^5 / 10^10 is a fraction (a decimal),
NOT a percentage yet. The question asks "what PERCENT," so we need to convert this fraction into a percentage by multiplying by
100.
p^5 / 10^10 × 100 = p^5 / 10^8So the answer is
p^5 / 10^8 %, which is choice
C.
Quick check with easy numbers: Let p =
100 (meaning
100% at every step — every galaxy qualifies at each stage). Then
100% of all galaxies should have sent a spaceship. Plugging in:
100^
5 /
10^
8 =
10^
10 /
10^
8 =
100%. That checks out!
The common mistake is stopping at p^5 / 10^10 and picking choice D, forgetting that the question asks for a percent, not a plain fraction.Answer: C