Let's suppose that 100,000 schoolchildren are tested, and out of these 100 actually have white lung, if 1 in 1000 children has white lung.
Therefore we have 99,900 who do not have white lung.
Of the 100 who actually have white lung, the test results will be positive for 97 children and negative for 3 children.
Of the 99,900 who do not have white lung, the test results will be positive for 3,996 (4% of 99,900) and negative for 95,904 children (96% of 99,900).
So the total number testing positive=97+3996=4,093, and negative=3+95904=95,907.
4,093 is 4.093% of 100,000 and 95,907 is 95.907% of 100,000.
So a child who actually has white lung is 97/4093=0.0237 approx of those testing positive. The probability of a child with white lung testing positive is 0.0237, outweighed by those children without white lung testing positive.