Quick way of doing HALF LIFE when given a different percentage of decay than 50?

[ThirdEye]

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I was doing a BR passage and it gave that there was 20% decay in 2 hours. The question asked how much would be left after 12 hours.

Of course I know how to do it:

Lets say 100g initially --->80 --> 64 --> 51.2 --->etc

The problem is with the rounding. If you're too liberal with the rounding you can serioulsy over or under estimate. This problem took me at least 3 min to solve and I just can't have that.

After looking back at the answers, I saw that this particular BR passage did give the half life (6hrs). So that makes it a lot easier of course, but I can't afford to spend minutes searching throgh a passage to see if a true half life is listed, especially if a risk coming out dry.

Anyone know of a formula or trick?

 

[zoner]

I was doing a BR passage and it gave that there was 20% decay in 2 hours. The question asked how much would be left after 12 hours.

After looking back at the answers, I saw that this particular BR passage did give the half life (6hrs). So that makes it a lot easier of course, but I can't afford to spend minutes searching throgh a passage to see if a true half life is listed, especially if a risk coming out dry.

Anyone know of a formula or trick?

I don't get how it can decay 20 % in 2 hours when it also said that the half life is 6 hrs.

 

[ThirdEye]

I don't get how it can decay 20 % in 2 hours when it also said that the half life is 6 hrs.

100g initially --->80 --> 64 --> 51.2 --->etc


It's an estimation. It goes from 100g to 51.2g in 6 hours

 

[zoner]

should be pretty straight forward. if half life is 6 hours and you start with 100g, after 12 hours, you have 25 grams left if it follows first order, i think....

you have to see it in the passage or at lease skim through the numbers at least. knowing 1/2 life is essential for this types of questions i think...