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## 'At Least One...' printed from http://nrich.maths.org/

Imagine flipping a coin three times.

What's the probability you will get a head on

at least one of the flips?

Charlie drew a tree diagram to help him to work it out:

He put a tick by all the outcomes that included at least one
head.

How could Charlie use his tree diagram to work out the probability
of getting

at least one
head?

How could he use it to work out the probability of getting no
heads?

What do you notice about these two probabilities?

Devise a quick way of working out the probability of getting

at least one head when
you flip a coin 4, 5, 6... times.

What is the probability of getting

at least one head when
you flip a coin ten times?

************************

Once you've worked out a neat strategy for the coins problem, take
a look at these related questions which can be solved in a similar
way:

Imagine choosing a ball from this bag and then replacing it.

If you did this three times, what's the probability that you
would pick at least one
green ball?

What if you didn't replace the ball each time?

Imagine a class with 15 girls and 13 boys.

Three children are chosen at random to represent the class at
School Council

What is the probability that there will be

at least one boy?

Why not try the problem

Same Number!
next?