There are 5 green marbles among the 10 in the bag, so the probability of drawing one at random is 5/10 = 0.50.
The odds in favor of drawing a green marble will be the ratio of the number of green marbles to the number that are not green: 5 : 5. Usually, the odds are expressed using integers with no common factors, so they would be written as 1 : 1.
The equation of line a is obtained by multiplying the equation of line b by 4. i.e. there are linearly dependent and hence there are infinitely many solutions to the set of equations.
