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King's Maths School

Challenge 440: Sum Probability

Give it a go - there's definitely sum probability you'll be successful!

I have a deck of 11 cards, which have the integers from 1 to 11 on them.

(a) I shuffle the cards and draw the top four. What is the probability that the sum is odd?

(b) I shuffle the cards and draw the top three. I add the first two, then subtract the third. What is the probability that the result is a multiple of 3?

Submit your solution

Please do send in your solution to this problem to weeklymaths@kcl.ac.uk You can scan or photograph your written work, or type your solutions. If this is your first weekly maths challenge solution, please include your year group and the name of the school you attend. We'll be happy to provide feedback on your solution, assuming that you are in year 11 or below. If you are older than this, we hope you enjoy trying the problems and reviewing your solutions against those we publish on the website.