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Bunuel
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Hi Bunuel , could you please explain why we took 500/124 and not (500-124)/124? Also, could you elaborate on when we should be taking a/b vs a-b/b

Thank you
Bunuel
Official Solution:

According to the new estimates, the number of stars in the Milky Way galaxy has increased from 124 billion to 415 billion, while the number of stars in the Andromeda galaxy has doubled. With these updated figures, the number of stars in the Milky Way is still only 42% of the number of stars in the Andromeda galaxy. Prior to the new estimates, approximately how many times more stars did the Andromeda galaxy have compared to the Milky Way galaxy?

A. 2
B. 4
C. 5
D. 8
E. 16


We are tasked with approximating, so let's proceed with that approach.

Consider using 400 billion instead of 415 billion, and 40% instead of 42%. This gives us: 400 billion = 0.4*(Andromeda's new estimate). Notice that the first approximation decreases the new estimate for the Andromeda galaxy, while the second approximation increases it. Therefore, these two approximations roughly balance each other out. Thus, from 400 billion = 0.4*(Andromeda's new estimate), we determine that Andromeda's new estimate is 1,000 billion. Given that the new estimates have doubled the numbers for the Andromeda galaxy, its old number was 500 billion.

Consequently, prior to the new estimates, the Andromeda galaxy had \(\frac{500}{124} \approx 4\) times more stars than the Milky Way galaxy.


Answer: B
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Hi Bunuel , could you please explain why we took 500/124 and not (500-124)/124? Also, could you elaborate on when we should be taking a/b vs a-b/b

Thank you


When you ask “how many times more stars did Andromeda have”, you compare the full Andromeda amount to the Milky Way amount. That is why the fraction is Andromeda_old divided by MilkyWay_old, which is 500/124.

If you do (500 - 124)/124, that gives the percent increase, not the number of times larger.
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