Question 1027097
 
Question:
A bag contains seven red marbles three blue marbles and one green marble. Find P (not blue)
 
Solution:
The bag contains only blue marbles, and either red or green marbles.
If the marble is not blue, it must be either red or green.
There are 3 blue marbles.
There are (7+1)=8 either red or green marbles
Total number of marbles = 3+8 =11
 
Assuming random pick of the marbles and that all marbles have equal probability of being picked, then 
P(not blue) = P(red or green) = number of red/green marbles  / total number of marbles.
 
I will leave you to replace the right-hand side with numbers to finish the answer.