Question

A bag contains 10 marbles. Four of them are red, three blue, two white and one yellow. A marble is drawn at random. What is the probability it is not blue?

Answer 1

Probability =
(number of different successful results) / (number of all possible results) .

Number of marbles that are not blue = 7
Number of marbles in the bag = 10
Probability that one marble drawn at random is not blue = 7/10 = 70% .

Answer 2

For all probability questions of this type, the probability can be found as:
$P_{desired}=\frac{desired}{total}$

or, more simply:
$P_{x}=\frac{n_{x}}{n_total}$

In this case:
$P_{notblue}=\frac{notblue}{total}$
$P_{notblue}=\frac{red+white+yellow}{red+blue+white+yellow}$
$P_{notblue}=\frac{4+2+1}{4+3+2+1}$
$P_{notblue}=\frac{7}{10} = 70\%$