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