# Coin probability problem!

• March 18th 2010, 08:46 AM
BabyMilo
Coin probability problem!

A fair coin is flipped 60 times, what is the percent chance of flipping heads 25 or fewer times?

would you solve it this way?

Quote:

binomal
B-(60,0.5)
P(X< or = 25)
so it's 60Cr * 0.5^(r) * 0.5^(60-r)
where r=0,1,2,3,4,5,......23,24,25

then that times 100 to get %.

=12.25%
• March 18th 2010, 02:12 PM
Quote:

Originally Posted by BabyMilo

A fair coin is flipped 60 times, what is the percent chance of flipping heads 25 or fewer times?

would you solve it this way?

Yes,

that would be right.

You could also use the Normal approximation to the Binomial.

$\mu=np=30$

$\sigma=\sqrt{npq}=\sqrt{15}$

$Z=\frac{25-30}{\sqrt{15}}=-1.29$

Applying the correction factor $\frac{0.7655}{\sqrt{np}}=0.14$

$Z=-1.29+0.14=-1.15$

which corresponds to 12.5%
• March 18th 2010, 02:51 PM
BabyMilo
Quote:

Yes,

that would be right.

You could also use the Normal approximation to the Binomial.

$\mu=np=30$

$\sigma=\sqrt{npq}=\sqrt{15}$

$Z=\frac{25-30}{\sqrt{15}}=-1.29$

Applying the correction factor $\frac{0.7655}{\sqrt{np}}=0.14$

$Z=-1.29+0.14=-1.15$

which corresponds to 12.5%

no idea what you did there.
but thanks for confirming.
• March 18th 2010, 03:18 PM