Independent solution

How to solve this Survival Functions question

Setup

Setup

Let T_i be the failure age of unit i. The supplied distribution gives the survival probability through age 2 directly.

Pr(Ti>2)=1F(2)=21/4\Pr(T_i>2)=1-F(2)=2^{-1/4}
Pr(T1>2,T2>2,T3>2)=(21/4)3\Pr(T_1>2,T_2>2,T_3>2)=\left(2^{-1/4}\right)^3

Model

Model

The payment indicator is one unless every unit avoids failure during the covered year.

I=1{at least one covered failure}I=\mathbf{1}\{\text{at least one covered failure}\}
Pr(I=1)=1Pr(T1>2,T2>2,T3>2)\Pr(I=1)=1-\Pr(T_1>2,T_2>2,T_3>2)

Compute

Compute

Use independence, take the complement, and multiply by the fixed benefit.

Pr(I=1)=123/4=0.4053964425\Pr(I=1)=1-2^{-3/4}=0.4053964425
E[1000I]=1000(0.4053964425)=405.3964425\operatorname{E}[1000I]=1000(0.4053964425)=405.3964425

Answer

Answer

The expected payment is approximately 405.

405(C)\boxed{405\quad\text{(C)}}