Simple Binomial Distribution question (need clarification)

Hi
the following question i have done, but my answer is incorrect.

A company receives 70% of its orders over the internet. On a particular day it receives 32 independent orders. Calculate the probability that between 20 and 30 (both inclusive) of the orders are received over the internet?

This is what i have done

Pr(20 <= X <= 30) = Pr(X <=30) - Pr(X <= 20) = binomcdf(32,.7,30)-binomcdf(32,.7,20) = .7715

However when i done binomcdf(32,.7,30)-binomcdf(32,.7,19) = 0.864 which is the correct answer, but don't understand how this is correct.

according to the question if "both inclusive" shouldn't it be Pr(20 <= X <= 30) ?

P.S

Quote:

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Originally Posted by Paymemoney

Hi
the following question i have done, but my answer is incorrect.

A company receives 70% of its orders over the internet. On a particular day it receives 32 independent orders. Calculate the probability that between 20 and 30 (both inclusive) of the orders are received over the internet?

This is what i have done

Pr(20 <= X <= 30) = Pr(X <=30) - Pr(X <= 20) = binomcdf(32,.7,30)-binomcdf(32,.7,20) = .7715

However when i done binomcdf(32,.7,30)-binomcdf(32,.7,19) = 0.864 which is the correct answer, but don't understand how this is correct.

according to the question if "both inclusive" shouldn't it be Pr(20 <= X <= 30) ?

P.S

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You want:

Pr(20 <= X <= 30) = P(X=30)+...+P(X=20)

......... =[P(X=30)+..+P(X=0)] - [P(X=19)+...+P(X=0)]

......... = Pr(X <=30) - Pr(X <= 19)

CB

but why is it from 19->0 shouldn't it be 20->0 if your saying including 20?

Quote:

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Originally Posted by Paymemoney

but why is it from 19->0 shouldn't it be 20->0 if your saying including 20?

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The complement of is
You want to include 20 and exclude 19 and below.

i understand now thanks