Bunuel
At the start of Potions class, Hermione and Ron are each given a magical cauldron. Hermione’s cauldron contains 27 magic beans, and Ron’s contains 729. At the end of every 6 hours, the number of beans in Hermione’s cauldron instantly increases by 200 percent, and at the end of every 18 hours, the number of beans in Ron’s cauldron instantly increases by 800 percent. If no beans are taken from either cauldron, after how many hours from the start of the class will the two cauldrons first contain the same number of beans?
A. 18
B. 27
C. 30
D. 36
E. 54
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For Hermione - Bean will increase by 200% i.e. it will become 3 times. Similarly for Ron a 800% increase will mean 9 times after 18 years.
at 6, 12, 18, 24, 30, 36 hours ...H changes to= 27* 3ˆ(n) ; n will be 1,2,3,4,5, respectively.
at 18, 36, 54 hours, R changes to = 729* 9ˆm ; m will be 1, 2, 3 respectively.
Clearly our answer will be a multiple of 6.
So from options, we check at 18 hours, then 30 hours, etc
at 18 hrs: H=27*3ˆ3 = 729. while R=729*9ˆ1
at 30 hrs: H=27*3ˆ5 =729*3ˆ2, while R=729*9ˆ1.
Thus ans = 30 hours
C.