its not 1/6. the answer is 50

come to think of it logically
i'll represent original content
with this line below
------------------(no evap)
------------ (rem 2/3rd after one day)
--- (rem 1/4th of previous 2/3rd
at end of 2nd day i.e. 1/6 is left)

i'd say the result doesn't resemble 50 ?
the 1/2 u r calculating is actually the amt evaporated
on 2nd day 2/3 - [(3/4)*(2/3)] 1/2 of 'original' content.

1/6 is right.....

First day it is 1/3 tat gets evaporated...then after 1st day we have 1-1/3 remaining ie. 2/3 and on the 2nd day 3/4 of the remaining (2/3) gets evaporated ie.(2/3*3/4))

lets calculate the content evaporated first.....

1/3 + (2/3*3/4) 1/3 + 1/2 5/6.......

Therefore the remainig content is 1 - 5/6 1/6....