Answer
$0.8$ or $80\%$
Work Step by Step
Find the total number of marbles
$50+30+20=100$
There are $20$ white marbles.
Therefore the number of marbles that are not white is:
$100-20=80$ marbles
Thus, the probability of choosing a non-white marble is
$\dfrac{\text{number of non-white marbles}}{\text{total number of marbles}}$=$\dfrac{80}{100}=\dfrac{4}{5}=0.8=80\%$
