K people of 500 people are divided into one group, * * * has 500/k groups of random variables, let Xi be the number of tests of the first group, and I = 1, 2,3,4 .......... 500/k.

p(0.7^k = 1)= Xi p(Xi = k+ 1)= 1-0.7^k

So e (xi) =1* 0.7k+(k+1) * (1-0.7k) =1+(1-0.7k) * k.

So e (x1+x2+x3+... x500/k) = 500/k (1+(1-0.7k) * k).

Sao nian, your answer 500/k [1* (0.7k)+(1-0.7k) * k] is wrong.