easy half life problem i can't do!

[msu08]

If the half-life of an isotopic nuclide is 24 hours, how long will it take for 75% of the sample to decay?



A. 12 hours
B. 24 hours
C. 36 hours
D. 48 hours
E. 60 hours

The answer is 48... i got 36..anyone understand this problem?

 

[zuma35]

Start with 100g. After 24 hours 50g remains. After 48 hours (another half life of 24 hours), 25g remains.

 

[montag925]

any halflife problem that I ever reviewed or did on the DAT the two times that I took it did not require any equation, but rather only common sense and reasoning as in this one.

 

[joonkimdds]

I know exactly what mistake u r making. U skipped 1 step.
U r going directly from 100% to 25%.
Just like a math problem.
0 ==> 50 miles takes 24 hrs.
how long will it take for u to go from 0 to 75 mile and answer would be 36 hrs.
But that's not the way to solve this problem.
Above method is assuming that ur speed never change.
but think about a crappy car that slows down speed in half after half life