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problem:After masks are produced by the factory, they appear in a test center, where they are tested for defects. Historically, only 10% of masks get a positive test result (meaning they are defective).

Let T1= number of masks tested until first positive result, T2= number of masks tested until second positive result.

E[T2∣T1=n].

i assume i have to use forumla: E(X∣Y=y)=∑xP(X=x∣Y=y)

but i dont quite understand how to calculate it

Silvija
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    Assuming each mask is defective independent of other masks, do you think the number of additional masks after the first mask is detected would depend on how many masks it took to detect the first one? – sudeep5221 May 21 '20 at 21:20
  • i think its independent – Silvija May 21 '20 at 22:38
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    You've already posted this question here: https://math.stackexchange.com/questions/3685608/approach-to-solve-conditional-probability – zipirovich May 21 '20 at 22:58
  • no it didn't and we actually discussed another part of this exercise there – Silvija May 21 '20 at 23:06

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