Suppose 100 people are waiting for a blood test for a kind of disease. The probability that for one person the test result is positive equals 0.1. Two test methods are as follows: Method 1: Test each person one by one. Then we have to do 100 tests. Method 2: Divide 100 people into 10 groups with 10 people in each group. Then, mix 10 people's blood and test that mixture. If the result is negative, then everyone in this group is negative. If the result is positive, then we test these 10 people's blood one by one. Now, the number of tests becomes l+lO=l 1. Question: On the average, what is the number of the tests we need to do by the second method