# A radioactive substance decays at a rate of 30% per year. Presently there are 400 grams of the substance. How long before there are 10 grams?

##### 1 Answer

May 28, 2016

#### Explanation:

If x gm is present at time t years,

So,

Initially, t = 0 and x = 400. So, C=400, and now,

The time t years for decay, from 400 gm to 10 gm, is given by

So, t=ln 40/0.3=12.296 years= 12 years and 108 days, nearly.