Out of 21 tickets marked with numbers from 1 to 21, three are

drawn at random. Find the probability that the numbers on them are in Arithmetic Progression?

I can understand that there are C(21,3) outcomes.But how many A.P series can be there?

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- Dec 6th 2011, 08:35 PMearthboyprobability of picking A.P series.
Out of 21 tickets marked with numbers from 1 to 21, three are

drawn at random. Find the probability that the numbers on them are in Arithmetic Progression?

I can understand that there are C(21,3) outcomes.But how many A.P series can be there? - Dec 7th 2011, 03:21 AMmr fantasticRe: probability of picking A.P series.
- Dec 7th 2011, 09:00 AMSorobanRe: probability of picking A.P series.
Hello, earthboy!

Quote:

Out of 21 tickets marked with numbers from 1 to 21, three are drawn at random.

Find the probability that the numbers on them are in Arithmetic Progression

I can understand that there are C(21,3) outcomes.

But how many A.P series can be there?

There is no formula for your question.

I see no choice but to List them and Count them . . . and look for a pattern.

Sequences beginning with 1: .

Sequences beginning with 2: .

Sequences beginning with 3: .

. . . . . . . . . . . . . . . . . . . . . . . . . .

Sequences beginning with 17:.

Sequences beginning with 18:.

Sequences beginning with 19:.

Do you see a pattern?