Answer:
18.51 grams.
Explanation:
We have been given the decay rate
We have general equation for decay which is:
;

Here, we are given with initial sample which is a=300
b=1-13%
And t is 20 years.
We will subatitute the values in the formula we get:



After simplification we get y=18.51 grams.
The sample remain will be 18.51 grams.