Thread: Binomial Probability

In roulette, a steel ball is rolled into a wheel that contains 18 red, 18 black and 2 green slots. If the ball is rolled 25 times, find the probabilities of the following events.

a) The ball falls into the green slots two or more times
b) The ball does not fall into any green slots
c) The ball falls into black slots 15 or more times
d) The ball falls into red slots 10 or fewer times


a) so n = 25 .. p = 2/38 and i calculated for x = 0 and x = 1 and added them up and subtracted it from 100% to get 71%.
b) 36/38 = 95%

Am i doing this right?
Can someone help me with c and d, thank you

Originally Posted by xfyz

In roulette, a steel ball is rolled into a wheel that contains 18 red, 18 black and 2 green slots. If the ball is rolled 25 times, find the probabilities of the following events.

a) The ball falls into the green slots two or more times
b) The ball does not fall into any green slots
c) The ball falls into black slots 15 or more times
d) The ball falls into red slots 10 or fewer times


a) so n = 25 .. p = 2/38 and i calculated for x = 0 and x = 1 and added them up and subtracted it from 100% to get 71%.
b) 36/38 = 95%

Am i doing this right?
Can someone help me with c and d, thank you

ok, so we are using the binomial distribution, for which the formula is, i'm sure you're familiar with what the variables mean, so i won't define them,

so for this set of questions, n = 25 always.

for (a), you want , here we have p = 2/38. you got that i think. hope your calculations are correct

for (b) you're not correct. here we want , p = 2/38 also here.

for (c) you want . here p = 18/38

for (d) you want . here p = 18/38

Originally Posted by Jhevon

ok, so we are using the binomial distribution, for which the formula is, i'm sure you're familiar with what the variables mean, so i won't define them,

so for this set of questions, n = 25 always.

for (a), you want , here we have p = 2/38. you got that i think. hope your calculations are correct

for (b) you're not correct. here we want , p = 2/38 also here.

for (c) you want . here p = 18/38

for (d) you want . here p = 18/38

for c and d, do I have to work out each one? For ex: c, do I have to tediously calculate P(15) + p(16) + ... and so on till p(25) all by hand? Is there a faster way?
thank you

Originally Posted by xfyz

for c and d, do I have to work out each one? For ex: c, do I have to tediously calculate P(15) + p(16) + ... and so on till p(25) all by hand? Is there a faster way?
thank you

Tedious calculations OR look it up in binomial distribution table

Originally Posted by xfyz

for c and d, do I have to work out each one? For ex: c, do I have to tediously calculate P(15) + p(16) + ... and so on till p(25) all by hand? Is there a faster way?
thank you

as Colby suggested, i guess you could try looking this up in a table. but there is really no other shorter way to do this. the other way would be 1 - P(0) - P(1) - ... - P(14), but that would actually be more calculations.

there are approximations to the binomial distribution, but i doubt they would be very accurate with n being so small.

so, i'm guessing the best way for you is to just grit your teeth and plow through it. if you have a calculator which you can save a formula in, it would make life easier for you