Let g be the number of grandchildren, and d be the number of dogs. They both have only one head, so the number of heads is
Grandchildren have 2 legs, while dogs have 4. So, the total number of legs is
From the first equation (the one for the heads) we can derive
Substitute this into the second to get
So, there are 9 dogs and (from the first equation
The bag contains,
Red (R) marbles is 9, Green (G) marbles is 7 and Blue (B) marbles is 4,
Total marbles (possible outcome) is,
Let P(R) represent the probablity of picking a red marble,
P(G) represent the probability of picking a green marble and,
P(B) represent the probability of picking a blue marble.
Probability , P, is,
Probablity of drawing a Red marble (R) and then a blue marble (B) without being replaced,
That means once a marble is drawn, the total marbles (possible outcome) reduces as well,
Hence, the best option is G.