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Probability, SM-Bank 028

Peter has a marble bag that contains 20 marbles that are either red or green in colour.

The probability of randomly picking a green marble is 70%.

  1. What is the probability of randomly picking a red marble?  (1 mark)
  2. How many red marbles are in the bag?  (1 mark)
Show Answers Only

a.    \(30\%\)

b.    \(6\)

Show Worked Solution
a.    \(P\text{(green)} + P\text{(red)}\) \(=100\%\)
\(70\% + P\text{(red)}\) \(=100\%\)
\(\therefore\ P\text{(red)}\) \(=30\%\)

 

b.    \(\text{Red marbles}\) \(=20\times 30\%\)
  \(=20\times \dfrac{3}{10}\)
  \(=6\)