Build the bag
A bag has n red, 7 white and 5 blue marbles. Add 9 red. Then P(red) = 7/11.
Before
6red
7 white5 blue
After adding 9
15red
27 totaltarget 7/11
P(red) =1527≈ 0.556Not 7/11 yet
Bag after +9 red
21 red7 white5 blue
33 totalSecond draw
21 red7 white5 blue
33 totalTwo red marbles:21/33 × ?/?Draw the first red marble to update the bag.
My reasoning trail
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PART A · 1 MARK
Model the new bagFind the value of n.
n + 9n + 21=711
×
Check both parts to finish.