BMAT 2018 Section 2 Question 8

My wallet contains two 50p coins and five 20p coins only. I need 70p to buy an item. If I pick two coins at random from my wallet, what is the probability that I have picked the exact money to buy the item?

Answer:

We need exactly 70p worth of coins. So that is one 50p coin and one 20p coin. Best way to do this is to draw a tree diagram.

We can only take the dashed paths shown above.

1st dashed path gives us = 2/7 x 5/6 = 10/42 = 5/21
2nd dashed path gives us = 5/7 x 2/6 = 10/42 = 5/21

Total = 5/21 + 5/21 = 10/21

So the answer is C.

Sami Qamar

I’m Sami Qamar. I’m a YouTuber, Blogger, and first year med student.

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